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

docs/DESIGN.md

@@ -1,6 +1,6 @@
 # Design decisions
 
-This file contains a log of miscellaneous design decisions in genqlient.
+This file contains a log of miscellaneous design decisions in genqlient.  They aren't all necessarily up to date with the exact implementation details, but are preserved here as context for why genqlient does X thing Y way.
 
 ## Types