CONTRIBUTING: initial draft

Signed-off-by: Cedric Hombourger <[email protected]>

CONTRIBUTING.md

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+Contributing to MTDA 
+====================
+
+Contributions to MTDA are always welcome. This document explains the
+general requirements on contributions and the recommended preparation
+steps. It also sketches the typical integration process of patches.
+
+
+Contribution Checklist
+----------------------
+
+- use git to manage your changes [**required**]
+
+- follow the existing coding style (run `pycodestyle` on changed files) [**required**]
+
+- add the required copyright header to each new file introduced, see
+  [licensing information](COPYING) [**required**]
+
+- structure patches logically, in small steps [**required**]
+    - one separable functionality/fix/refactoring = one patch
+    - do not mix those there in a single patch
+    - after each patch, the tree still has to build and work, i.e. do not add
+      even temporary breakages inside a patch series (helps when tracking down
+      bugs)
+    - use `git rebase -i` to restructure a patch series
+
+- base patches on top of latest master or - if there are dependencies - on next
+  (note: next is an integration branch that may change non-linearly)
+
+- test patches sufficiently (obvious, but...) [**required**]
+    - no regressions are caused in affected code
+    - the world is still spinning
+
+- add signed-off to all patches [**required**]
+    - to certify the "Developer's Certificate of Origin", see below
+    - check with your employer when not working on your own!
+
+- create a pull request on GitHub
+
+- post follow-up version(s) if feedback requires this
+
+- send reminder if nothing happened after about a week
+
+
+Developer's Certificate of Origin 1.1
+-------------------------------------
+
+When signing-off a patch for this project like this
+
+    Signed-off-by: Random J Developer <[email protected]>
+
+using your real name (no pseudonyms or anonymous contributions), you declare the
+following:
+
+    By making a contribution to this project, I certify that:
+
+        (a) The contribution was created in whole or in part by me and I
+            have the right to submit it under the open source license
+            indicated in the file; or
+
+        (b) The contribution is based upon previous work that, to the best
+            of my knowledge, is covered under an appropriate open source
+            license and I have the right under that license to submit that
+            work with modifications, whether created in whole or in part
+            by me, under the same open source license (unless I am
+            permitted to submit under a different license), as indicated
+            in the file; or
+
+        (c) The contribution was provided directly to me by some other
+            person who certified (a), (b) or (c) and I have not modified
+            it.
+
+        (d) I understand and agree that this project and the contribution
+            are public and that a record of the contribution (including all
+            personal information I submit with it, including my sign-off) is
+            maintained indefinitely and may be redistributed consistent with
+            this project or the open source license(s) involved.
+
+
+Contribution Integration Process
+--------------------------------
+
+1. patch reviews performed on GitHub
+    * at least by maintainers, but everyone is invited
+    * feedback has to consider design, functionality and style
+    * simpler and clearer code preferred, even if original code works fine
+
+2. accepted patches merged into next branch
+
+3. further testing done by community
+
+4. if no new problems or discussions showed up, acceptance into master
+    * grace period for master: about 3 days
+    * urgent fixes may be applied sooner