doc: add README

The README will appear on go.googlesource.com/scratch and show people
how to get started contributing to this repository.

Change-Id: I4b134071cda1c00e22ca53fcc233d9f0245930d2
Reviewed-on: https://go-review.googlesource.com/49450
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

README.md

@@ -0,0 +1,51 @@
+# Scratch Repository
+
+This repository is mainly for use by people learning how to use Gerrit and
+contribute to Go.
+
+[Click here for a tutorial][gophercon-tutorial] on how to get started with a
+contribution to this repository.
+
+A fuller, text-based tutorial based around the [core Go project can be found
+here][core-go-tutorial].
+
+## What should I add?
+
+Add a folder with your username, and put a main function in there. You can
+put whatever you want in your main function; see the existing directories for
+examples.
+
+All files should have the standard licensing header, and add appropriate
+documentation see the other files in this repository for an example.
+
+## Notes about Gerrit
+
+If you have needed to change a Github pull request, you probably just added a
+second commit with the requested changes and pushed it. By contrast, all changes
+opened in Gerrit are a single commit, which means you need to [amend your
+commit][amend] if the reviewer requests feedback.
+
+[amend]: http://www.joinfu.com/2013/06/pushing-revisions-to-a-gerrit-code-review/
+
+To amend a previous commit, run `git add (list of files you changed)` to add
+your changes, then run `git commit --amend` to amend the commit to add new
+data. Your commit message should still summarize the entire commit ("Add
+kevinburke/main.go"), not just the change a reviewer asked for ("Fix typo").
+
+After you amend the commit, re-run `git codereview mail` to push that change
+to the server. Then in the Gerrit UI, find the in-line comments left by your
+reviewer, click "Done", go back to the main PR page, and click "Reply" => "Send"
+to tell your reviewer that you've addressed your feedback.
+
+Once you get a "Code-Review: +2" from a Go contributor, your change will be
+merged!
+
+## Need help?
+
+Gerrit is not easy to get started with, and we want to help you out. If you are
+having trouble with Gerrit, contact the [golang-devexp][devexp] mailing list for
+help!
+
+[gophercon-tutorial]: https://golang.org/s/gophercon2017
+[core-go-tutorial]: https://golang.org/doc/contribute.html
+[devexp]: https://groups.google.com/forum/#!forum/golang-devexp