Fix links, typos & formatting in markdown docs

 - Major cleanup to documentation
 - CAF_ABI.md needs further attention, esp. TODO lists
 - Added a proper CONTRIBUTING.md
 - Still need to add:
   - Developer guide for SI devs
   - CHANGELOG.md

Author: zbeekman

AUTHORS.md

@@ -1,7 +1,14 @@
-# Contributors to OpenCoarrays
+Contributors to OpenCoarrays
+============================
 
  - Alessandro Fanfarillo  <fanfarillo@ing.uniroma2.it>
  - Damian Rouson  <damian@rouson.net>
+ - Izaak Beekman  <http://izaakbeekman.com>
  - Daniel Carrera  <dcarrera@gmail.com>
  - Jeff Hammond  <jeff.science@gmail.com>
  - Tobias Burnus  <burnus@net-b.de>
+
+[I think we're missing a number of people here]:#
+[@naveen-rn?]:#
+[@raul-nasner?]:#
+[Others:]:#