Reorganize documentation to make room to grow (#84)

## Summary:
In this commit I reorganize much of our documentation into a new `docs`
directory, where there will hopefully be more room to grow and to
organize things in a user-friendly way.  There's almost no net-new
documentation, although of course it's a great time to review it anyway.

In particular:
- I moved the documentation for the `genqlient.yaml` config file into an
  example file instead of GoDoc (which now just points to the example
  file); I think this will be a lot clearer for casual users.
- I moved the documentation for the `@genqlient` directive out of GoDoc
  and into a GraphQL schema file (since while it's a comment it's all
  real syntax), likewise, and made the `GenqlientDirective` type private
  (since there's now nothing useful to do with it).
- I moved `DESIGN.md` and the logo into `docs/` (just to keep the
  toplevel a bit cleaner), and separated the Contributing section of the
  README into `docs/CONTRIBUTING.md` (which github will automatically
  link on various issue and PR pages).

This leaves it so that:
- README.md is the only documentation at the toplevel (and will become
  just the high-level introduction as I add more user docs to `docs/`)
- GoDoc is only documentation for if you want to call genqlient
  programmatically (which is fairly limited as the API surface is quite
  small: it's now just Main, Generate, and Config, plus a constructor, a
  single method, and a bunch of fields on the latter)

In future commits, I'll add some more new documentation to the `docs`
directory.

Issue: https://github.com/Khan/genqlient/issues/26

## Test plan:
make check (and read the docs)


Author: benjaminjkraft

Reviewers: jvoll, benjaminjkraft, aberkan, dnerdy, MiguelCastillo, mahtabsabet

Required Reviewers: 

Approved By: jvoll

Checks: ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/84

docs/genqlient_directive.graphql

@@ -0,0 +1,86 @@
+# The quasi-directive @genqlient is used to configure genqlient on a
+# query-by-query basis.
+#
+# The syntax of the directive is just like a GraphQL directive (as defined
+# below), except it goes in a comment on the line immediately preceding the
+# field.  (This is because GraphQL expects directives in queries to be defined
+# by the server, not by the client, so it would reject a real @genqlient
+# directive as nonexistent.)
+#
+# Directives may be applied to fields, arguments, or the entire query.
+# Directives on the line preceding the query apply to all relevant nodes in
+# the query; other directives apply to all nodes on the following line.  (In
+# all cases it's fine for there to be other comments in between the directive
+# and the node(s) to which it applies.)  For example, in the following query:
+#  # @genqlient(n: "a")
+#
+#  # @genqlient(n: "b")
+#  #
+#  # Comment describing the query
+#  #
+#  # @genqlient(n: "c")
+#  query MyQuery(arg1: String,
+#    # @genqlient(n: "d")
+#    arg2: String, arg3: String,
+#    arg4: String,
+#  ) {
+#    # @genqlient(n: "e")
+#    field1, field2
+#    field3
+#  }
+# the directive "a" is ignored, "b" and "c" apply to all relevant nodes in the
+# query, "d" applies to arg2 and arg3, and "e" applies to field1 and field2.
+directive genqlient(
+
+  # If set, this argument will be omitted if it's equal to its Go zero
+  # value, or is an empty slice.
+  #
+  # For example, given the following query:
+  #  # @genqlient(omitempty: true)
+  #  query MyQuery(arg: String) { ... }
+  # genqlient will generate a function
+  #  MyQuery(ctx context.Context, client graphql.Client, arg string) ...
+  # which will pass {"arg": null} to GraphQL if arg is "", and the actual
+  # value otherwise.
+  #
+  # Only applicable to arguments of nullable types.
+  omitempty: Boolean
+
+  # If set, this argument or field will use a pointer type in Go.  Response
+  # types always use pointers, but otherwise we typically do not.
+  #
+  # This can be useful if it's a type you'll need to pass around (and want a
+  # pointer to save copies) or if you wish to distinguish between the Go
+  # zero value and null (for nullable fields).
+  pointer: Boolean
+
+  # If set, this argument or field will use the given Go type instead of a
+  # genqlient-generated type.
+  #
+  # The value should be the fully-qualified type name to use for the field,
+  # for example:
+  #  time.Time
+  #  map[string]interface{}
+  #  []github.com/you/yourpkg/subpkg.MyType
+  # Note that the type is the type of the whole field, e.g. if your field in
+  # GraphQL has type `[DateTime]`, you'd do
+  #  # @genqlient(bind: "[]time.Time")
+  # (But you're not required to; if you want to map to some type DateList,
+  # you can do that, as long as its UnmarshalJSON method can accept a list
+  # of datetimes.)
+  #
+  # See bindings in genqlient.yaml for more details; this is effectively to a
+  # local version of that global setting and should be used with similar care.
+  # If set to "-", overrides any such global setting and uses a
+  # genqlient-generated type.
+  bind: String
+
+) on
+  # genqlient directives can go almost anywhere, although some options are only
+  # applicable in certain locations as described above.
+  | QUERY
+  | MUTATION
+  | SUBSCRIPTION
+  | FIELD
+  | FRAGMENT_DEFINITION
+  | VARIABLE_DEFINITION