doc: Update docs on contributing

CONTRIBUTING

@@ -1,11 +0,0 @@
-Contributing to c-ares
-======================
-
-The c-ares developers prefer patches to be sent to the c-ares mailing list
-rather than receiving pull requests via GitHub.  So for suggested changes
-please:
-
- - Subscribe to the mailing list at:
-     http://cool.haxx.se/mailman/listinfo/c-ares
- - Use 'git format-patch' to generate patch files.
- - Send the patches to the mailing list at c-ares@cool.haxx.se

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+Contributing to c-ares
+======================
+
+To contribute patches to c-ares, please generate a GitHub pull request
+and follow these guidelines:
+
+ - Check that the Travis builds are green for your pull request.
+ - Please update the test suite to add a test case for any new functionality.
+ - Build the library with `./configure --enable-debug --enable-maintainer-mode` and
+   ensure there are no new warnings.
+
+To improve the chances of the c-ares maintainers responding to your request:
+
+ - Also send an email to the mailing list at `c-ares@cool.haxx.se` describing your change.
+ - To follow any associated discussion, please subscribe to the [mailing list](http://cool.haxx.se/mailman/listinfo/c-ares).

README.md

@@ -59,3 +59,5 @@ Notes for c-ares hackers
 * Newlines must remain unix-style for older compilers' sake.
 
 * Comments must be written in the old-style /* unnested C-fashion */
+
+* Try to keep line lengths below 80 columns.