Merge: Added contributing guidelines and link from readme

As discussed in #2133, contribution guidelines are hard to find.

This PR is an attempt to ease the process by giving newcomer friendly advices and some more insight on the contribution process for Nit.

@tonygaetani, do you think a document like this one would have been helpful for you ? Is there anything you would change to make it friendlier/clearer ?

Close #2133

Pull-Request: #2134
Reviewed-by: Jean Privat <jean@pryen.org>

diff --combined README.md
index 1a5dd3c,826686a..2679f4d
--- 1/README.md
--- 2/README.md
+++ b/README.md
@@@ -18,7 -18,7 +18,7 @@@ Requirements
   * gcc                http://gcc.gnu.org/ (or a compatible C compiler)
   * pkg-config http://www.freedesktop.org/wiki/Software/pkg-config/
   * ccache     http://ccache.samba.org/        to improve recompilation
 - * libgc-dev  http://www.hpl.hp.com/personal/Hans_Boehm/gc/
 + * libgc-dev  http://hboehm.info/gc/
   * graphviz   http://www.graphviz.org/        to enable graphs with the nitdoc tool
   * libunwind  http://nongnu.org/libunwind
  
@@@ -65,6 -65,11 +65,11 @@@ To have your environment automatically 
  
      $ . misc/nit_env.sh install
  
+ Contributing:
+ 
+ To contribute to Nit, please see [CONTRIBUTING](CONTRIBUTING.md).
+ 
+ The best way to ask the team for advice, submit bugs or request features, is through the use of Github issues, using the appropriate tag (`forum`, `feature-request`, `bug`).
  
  Information, contacts and help: