README: add context on maintainers and a testscript overview with links (rogpeppe#225)

Help give some context for people who might be evaluating using this repo and are wondering:

* what is the bus factor for the repo?
* who are the maintainers?
* are the maintainer(s) of this repo still interested in this repo (including, are they still using it)?
* are other people using testscript?
* where did testscript come from?

Obviously, someone can hunt around to determine many of these things on their own,
but the intent is to help make that process more efficient.

Updates rogpeppe#196.

Author: thepudds

README.md

@@ -1,6 +1,15 @@
 This repository factors out an opinionated selection of internal packages and functionality from the Go standard
 library. Currently this consists mostly of packages and testing code from within the Go tool implementation.
 
+This repo is [primarily maintained](https://github.com/rogpeppe/go-internal/graphs/contributors) by long-time
+[Go contributors](https://github.com/golang/go/contributors) who are also currently
+[maintaining CUE](https://github.com/cue-lang/cue/graphs/contributors) (which is primarily written in Go
+and which relies upon several of the packages here).
+
+Contributions are welcome, but please open an issue for discussion first.
+
+## Packages
+
 Included are the following:
 
 - dirhash: calculate hashes over directory trees the same way that the Go tool does.
@@ -15,6 +24,22 @@ Included are the following:
 - testscript: script-based testing based on txtar files
 - txtar: simple text-based file archives for testing.
 
-# Links
+### testscript
 
-- [Test scripts in Go](https://bitfieldconsulting.com/golang/test-scripts)
+The most popular package here is the [testscript](https://pkg.go.dev/github.com/rogpeppe/go-internal/testscript) package:
+ * Provides a shell-like test environment that is very nicely tuned for testing Go CLI commands.
+ * Extracted from the core Go team's internal testscript package ([cmd/go/internal/script](https://github.com/golang/go/tree/master/src/cmd/go/internal/script)),
+ which is [heavily used](https://github.com/golang/go/tree/master/src/cmd/go/testdata/script) to test the `go` command.
+ * Supports patterns for checking stderr/stdout, command pass/fail assertions, and so on.
+ * Integrates well with `go test`, including coverage support.
+ * Inputs and sample output files can use the simple [txtar](https://pkg.go.dev/golang.org/x/tools/txtar)
+ text archive format, also used by the Go playground.
+ * Allows [automatically updating](https://pkg.go.dev/github.com/rogpeppe/go-internal/testscript#Params)
+ golden files.
+ * Built-in support for Go concepts like build tags.
+ * Accompanied by a [testscript](https://github.com/rogpeppe/go-internal/tree/master/cmd/testscript) command
+ for running standalone scripts with files embedded in txtar format.
+ 
+ A nice introduction to using testscripts is this [blog post](https://bitfieldconsulting.com/golang/test-scripts) series.
+ Both testscript and txtar were [originally created](https://github.com/golang/go/commit/5890e25b7ccb2d2249b2f8a02ef5dbc36047868b)
+ by Russ Cox.