Add a bunch more documentation (and rewrite README) (#86)

## Summary:
In this commit, I add a bunch more documentation of how to use
genqlient, including a getting started guide, and a FAQ that also acts
as a how-to shaped index of common configuration options and extension
points.  Much of this is adapted from the internal doc I wrote at Khan,
or reorganized from what was in the README.  Speaking of which, now that
there are better places for all the details, I rewrote the README to be
a bit more of an overview and index of other documentation.  (For now I
left the "unmaintained" notes, which I'll remove fairly soon once all
our ducks are in a row.)

Fixes #26, #39.

Issue: https://github.com/Khan/genqlient/issues/26
Issue: https://github.com/Khan/genqlient/issues/39

## Test plan:
make check, read the docs


Author: benjaminjkraft

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

Required Reviewers: 

Approved By: mahtabsabet

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

Pull Request URL: https://github.com/Khan/genqlient/pull/86

graphql/client.go

@@ -14,7 +14,8 @@ import (
 // Client is the interface that the generated code calls into to actually make
 // requests.
 //
-// Unstable: This interface is likely to change before v1.0, see #19.
+// Unstable: This interface is likely to change before v1.0, see #19.  Creating
+// a client with NewClient will remain the same.
 type Client interface {
 	// MakeRequest must make a request to the client's GraphQL API.
 	//