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

generate/names.go

@@ -2,7 +2,7 @@ package generate
 
 // This file generates the names for genqlient's generated types.  This is
 // somewhat tricky because the names need to be unique, stable, and, to the
-// extent possible, human-readable and -writable.  See DESIGN.md for an
+// extent possible, human-readable and -writable.  See docs/DESIGN.md for an
 // overview of the considerations; in short, we need long names.
 //
 // Specifically, the names we generate are of the form:
@@ -33,9 +33,9 @@ package generate
 // One subtlety in the above description is: is the "MyType" the interface or
 // the impelmentation?  When it's a suffix, the answer is both: we generate
 // both MyFieldMyInterface and MyFieldMyImplementation, and the latter, in Go,
-// implements the former.  (See DESIGN.md for more.)  But as an infix, we use
-// the type on which the field is requested.  Concretely, the following schema
-// and query:
+// implements the former.  (See docs/DESIGN.md for more.)  But as an infix, we
+// use the type on which the field is requested.  Concretely, the following
+// schema and query:
 //	type Query { f: I }
 //	interface I { g: G }
 //	type T implements I { g: G, h: H }