README: switch to Markdown and add 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/tools or https://github.com/golang/tools
figure out how to get started with development.

Change-Id: I778202d9759df5cf79e86b8524046e108aa3db3d
Reviewed-on: https://go-review.googlesource.com/49852
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: kevinburke

README

@@ -0,0 +1,27 @@
+# Go Tools
+
+This subrepository holds the source for various packages and tools that support
+the Go programming language.
+
+Some of the tools, `godoc` and `vet` for example, are included in binary Go
+distributions.
+
+Others, including the Go `guru` and the test coverage tool, can be fetched with
+`go get`.
+
+Packages include a type-checker for Go and an implementation of the
+Static Single Assignment form (SSA) representation for Go programs.
+
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/tools/...`. You can
+also manually git clone the repository to `$GOPATH/src/golang.org/x/tools`.
+
+## 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 tools repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/tools/(your
+subdir):" in the subject line, so it is easy to find.