README: switch to Markdown, add better links

Move the README to README.md so Gerrit can render it; currently
Gerrit only renders files named exactly "README.md" (for example at
https://go.googlesource.com/go).

Add more links to the README explaining how to file issues,
how to submit code changes, where to download the code to and
how to get it. Hopefully this should help people who go to
https://go.googlesource.com/sync or https://github.com/golang/sync
figure out how to get started with development.

Change-Id: Iafdbec407bd6f61459aabe6e2c4703132d038b69
Reviewed-on: https://go-review.googlesource.com/49872
Reviewed-by: Kevin Burke <[email protected]>

Author: kevinburke

README

@@ -0,0 +1,18 @@
+# Go Sync
+
+This repository provides Go concurrency primitives in addition to the
+ones provided by the language and "sync" and "sync/atomic" packages.
+
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/sync`. You can
+also manually git clone the repository to `$GOPATH/src/golang.org/x/sync`.
+
+## Report Issues / Send Patches
+
+This repository uses Gerrit for code changes. To learn how to submit changes to
+this repository, see https://golang.org/doc/contribute.html.
+
+The main issue tracker for the sync repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/sync:" in the
+subject line, so it is easy to find.