[🔥AUDIT🔥] Release v0.2.0 (#119)

🖍 _This is an audit!_ 🖍

## Summary:
With the blog post going up soon, and a few new features, it's time for
another release!  (Plus, Craig has run the webapp tests on this commit,
so it's now extra-tested.)  This commit updates the changelog, and I'll
tag it with the release once it lands.

## Test plan:
crossed fingers


Author: benjaminjkraft

Auditors: csilvers, dnerdy, StevenACoffman

Required Reviewers: 

Approved By: 

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/119

docs/CHANGELOG.md

@@ -22,6 +22,16 @@ When releasing a new version:
 
 ### Breaking changes:
 
+### New features:
+
+### Bug fixes:
+
+## v0.2.0
+
+Version 0.2.0 adds several convenience features for using custom scalars, as well as many internal improvements and bug fixes.
+
+### Breaking changes:
+
 - The [`graphql.Client`](https://pkg.go.dev/github.com/Khan/genqlient/graphql#Client) interface now accepts `variables interface{}` (containing a JSON-marshalable value) rather than `variables map[string]interface{}`.  Clients implementing the interface themselves will need to change the signature; clients who simply call `graphql.NewClient` are unaffected.
 - genqlient's handling of the `omitempty` option has changed to match that of `encoding/json`, from which it had inadvertently differed.  In particular, this means struct-typed arguments with `# @genqlient(omitempty: true)` will no longer be omitted if they are the zero value.  (Struct-pointers are still omitted if nil, so adding `pointer: true` will typically work fine.  It's also now possible to use a custom marshaler to explicitly map zero to null.)