github,README: add issue/pr templates and README explaining closure

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,4 @@
+### This repository is no longer actively maintained.
+
+Development of the packages in this repository has moved to https://github.com/gonum/gonum.
+Please file issues [there](https://github.com/gonum/gonum/issues) after having checked that your issue has not been fixed.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,4 @@
+### This repository is no longer actively maintained.
+
+Development of the packages in this repository has moved to https://github.com/gonum/gonum.
+Please send pull requests [there](https://github.com/gonum/gonum/pulls) after having checked that your addition has not already been made.

README.md

@@ -1,5 +1,7 @@
 # Gonum unit  [![Build Status](https://travis-ci.org/gonum/unit.svg)](https://travis-ci.org/gonum/unit)  [![Coverage Status](https://coveralls.io/repos/gonum/unit/badge.svg?branch=master&service=github)](https://coveralls.io/github/gonum/unit?branch=master) [![GoDoc](https://godoc.org/github.com/gonum/unit?status.svg)](https://godoc.org/github.com/gonum/unit)
 
+# This repository is no longer maintained. Development has moved to https://github.com/gonum/gonum.
+
 Package unit provides a set of types and constants that facilitate the use of the International System of Units (SI).
 
 ## Issues