README: flush out description

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/net or https://github.com/golang/net
figure out how to get started with development.

Change-Id: I4eb54f2b210e4f028c9f00955ff19b55643315e6
Reviewed-on: https://go-review.googlesource.com/49850Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

README.md

+# Go Networking
+
 This repository holds supplementary Go networking libraries.
 
-To submit changes to this repository, see http://golang.org/doc/contribute.html.
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/net`. You can
+also manually git clone the repository to `$GOPATH/src/golang.org/x/net`.
+
+## 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 net repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/net:" in the
+subject line, so it is easy to find.