Actually mark genqlient as ready to use! (Khan#102)

## Summary:
I'm not going to really publicize it until the blog post is done, but
with all the usual boilerplate set up, it's time to remove this note!
(Actually, I meant to do it in Khan#96, but forgot.)

## Test plan:
read it


Author: benjaminjkraft

Reviewers: dnerdy, dangoor, StevenACoffman

Required Reviewers: 

Approved By: dnerdy, dangoor

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: Khan#102

README.md

@@ -7,8 +7,6 @@
 
 # genqlient: a truly type-safe Go GraphQL client
 
-This is a proof-of-concept of using code-generation to create a truly type-safe GraphQL client in Go.  It is not yet ready for contributions (see [docs/CONTRIBUTING.md](docs/CONTRIBUTING.md)).
-
 ## What is genqlient?
 
 genqlient is a Go library to easily generate type-safe code to query a GraphQL API. It takes advantage of the fact that both GraphQL and Go are typed languages to ensure at compile-time that your code is making a valid GraphQL query and using the result correctly, all with a minimum of boilerplate.
@@ -25,7 +23,7 @@ You can download and run genqlient the usual way: `go run github.com/Khan/genqli
 
 ## How can I help?
 
-genqlient is not yet ready for contributions, but once it is, you'll be able to learn more in the ([Contribution Guidelines](docs/CONTRIBUTING.md)), or file an issue [on GitHub](issues).
+genqlient welcomes contributions!  Check out the ([Contribution Guidelines](docs/CONTRIBUTING.md)), or file an issue [on GitHub](issues).
 
 ## Why another GraphQL client?