README: add better links, Markdown

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

Change-Id: I2b3b9cb1021292c3cb4b5eb3a84b5c17eb8429da
Reviewed-on: https://go-review.googlesource.com/49911Reviewed-by: Kevin Burke <kev@inburke.com>

README

-This repository holds supplemental Go packages for low-level interactions with the operating system.
-
-To submit changes to this repository, see http://golang.org/doc/contribute.html.

README.md

+# sys
+
+This repository holds supplemental Go packages for low-level interactions with
+the operating system.
+
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/sys`. You can
+also manually git clone the repository to `$GOPATH/src/golang.org/x/sys`.
+
+## 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 sys repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/sys:" in the
+subject line, so it is easy to find.