patch 7.4.846

Problem:    Some GitHub users don't know how to use issues.
Solution:   Add a file that explains the basics of contributing.

CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing to Vim
+
+Patches are welcome in whatever form.
+Discussions about patches happen on the vim-dev maillist.
+If you create a pull request on GitHub it will be
+forwarded to the vim-dev maillist.  You can also send your patch there
+directly.  An attachment with a unified diff format is preferred.
+Information about the maillist can be found [on the Vim website].
+
+[on the Vim website]: http://www.vim.org/maillist.php#vim-dev
+
+Please consider adding a test.  Test coverage isn't very good yet, this needs
+to improve.  Look through recent patches for examples.  The tests are located
+under "src/testdir".
+
+
+# Reporting issues
+
+We use GitHub issues, but that is not a requirement.  Writing to the Vim
+maillist is also fine.
+
+Please use the GitHub issues only for actual issues. If you are not 100% sure
+that your problem is a Vim issue, please first discuss this on the Vim user
+maillist.  Try reproducing the problem without any plugins or settings:
+
+    vim -N -u NONE
+
+If you report an issue, please describe exactly how to reproduce it.
+For example, don't say "insert some text" but say what you did exactly:
+"ahere is some text<Esc>".  Ideally, the steps you list can be used to write a
+test to verify the problem is fixed.
+
+Feel free to report even the smallest problem, also typos in the documentation.
+
+You can find known issues in the todo file: ":help todo".
+Or open [the todo file] on GitHub to see the latest version.
+
+[the todo file]: https://github.com/vim/vim/blob/master/runtime/doc/todo.txt