CONTRIBUTING.md: move into .github

To hide github specific files somewhat from the rest.

.github/CONTRIBUTING.md

@@ -0,0 +1,25 @@
+How to contribute to curl
+=========================
+
+Join the community
+------------------
+
+ 1. Click 'watch' on the github repo
+
+ 2. Subscribe to the suitable [mailing lists](https://curl.haxx.se/mail/)
+
+Read [docs/CONTRIBUTE](docs/CONTRIBUTE)
+---------------------------------------
+
+Send your suggestions using one of these methods:
+-------------------------------------------------
+
+ 1. in a mail to the mailing list
+
+ 2. as a pull request on github
+
+ 3. as an issue on github
+
+
+/ The cURL team!
+