Updated contribution information to include Gerrit and code style

As patches must be contributed by means of Gerrit as decribed in [1],
the file CONTRIBUTING.md needs some guidance on this. Moreover, some
information on code style has been added as well.

[1] https://www.eclipse.org/projects/handbook/#resources-source

Change-Id: Ib415a548f39e3c6b5025d89873d7a85b71ea5d5c

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 72f4b64..01825f1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -45,3 +45,67 @@
 that contributions are always welcome!
 
 - https://bugs.eclipse.org/bugs/enter_bug.cgi?product=tinydtls
+
+Submit Patches via Gerrit:
+--------------------------
+
+Patches must follow to the tinydtls coding style and must be submitted
+to [gerrit](https://git.eclipse.org/r/p/tinydtls/org.eclipse.tinydtls)
+for review. To submit a patch to gerrit, the author needs to have a
+CLA on file and must have a Signed-off-by entry with the same email
+address in the commit message footer.
+
+- https://www.eclipse.org/projects/handbook/#resources-source 
+
+Every new file must contain the Eclipse license information and the
+copyright holder(s). Please take a look into existing files and adopt
+the needed changes to your new file(s).
+
+Tinydtls Coding style:
+----------------------
+
+* For better reading the indentation is set to 2 characters as spaces,
+  this is depended on the often used nested functions like
+  'if-else'. Don't use TABs any there! Avoid trailing white spaces at
+  the end of a line.
+
+* Single lines within the source code should not be longer then 78
+  characters.
+
+* In the implementation (i.e., in files ending with '.c'), function
+  identifiers start on the first column of a line. The function's
+  return type preceeds the function identifier on a line of its
+  own. For example, in `dtls.c` the following definition is found:
+
+```
+dtls_peer_t *
+dtls_get_peer(const dtls_context_t *ctx, const session_t *session) {
+...
+}
+```
+
+* Declarations in header files do not follow the previous rule. For
+  example, the declaration for `dtls_get_peer()` in `dtls.h` reads as
+  follows:
+
+```
+dtls_peer_t *dtls_get_peer(const dtls_context_t *context,
+			   const session_t *session);
+```
+
+* A useful source code documentation is mandatory. Mostly to be done
+  within the source code files.
+
+* Please set up/adjust the doxygen documentation if you create new
+  functions or change existing functions. The doxygen documentation
+  has to be done in the header files as they are the public part of
+  tinydtls and only use the @-syntax for doxygen commands (akin to
+  javadoc).
+
+* Never break the API!
+  Do not remove old functions unless absolutely necessary. If changes
+  are needed in some kind always provide a wrapper for the old call to
+  let the library be backward compatible and mark the old function as
+  @deprecated in the doxygen comment.  Please discuss needed changes
+  on the mailing list.
+