github: add a CONTRIBUTING.md file

Initial version that likely got a broken syntax :(

CONTRIBUTING.md

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+# Contribution requirement
+* be patient
+    Your contribution will gladly be reviewed but 
+* discuss with us your future contribution before coding it
+    Let's avoid duplicate work! Besides specification could be clarified this way.
+
+
+# Window contribution possibility
+* check linux chapter ;) Various improvements are actually cross platform!
+
+# Linux contribution possibility
+
+You're welcome to the [linux contribution thread](http://forums.pcsx2.net/Thread-Areas-of-interest-for-new-linux-developers) to have full details. Here an handy list of feature that you could implement. Feel free to propse new enhancement.
+
+## House keeping and general compilation
+* clean gcc flags
+* clean gcc warning
+* support clang (template mess)
+* speed comparison clang/gcc
+* support avx (gsdx)
+* add missing/update copyright header 
+* LTO support
+* PGO support
+
+## Core
+* support XZ compressed iso
+
+## GSdx
+* Fix OpenGL
+* implement DX features on OGL (Amsodean's fxaa/video recording ....)
+* Fix GLES3 
+* add tooltip on gsdx gui
+* finish shader subroutine usage (+find a way to clean shader and avoid duplication)
+* finish buffer storage
+* OSD
+
+## CDVD
+* port CDVDgiga to linux ?
+
+## zzogl
+* reduce gl requirement to 3.3 + gl4 extension
+* use multibind
+* fix EGL
+* port GLSL to window
+* Drop old GLSL backend (and much later Nivida CG)
+* support wx3.0
+
+## Portability
+* port GSThread to std::thread
+* port core thread to std::thread
+* C11 aligned_alloc
+* C++11 alignof/alignas syntax
+* replace volatile/lock-free queue with real C++ atomic
+
+## Debian package
+* need a refresh to the latest standard
+* Clean debian/copyright => debmake -k
+
+
+## QA
+* [C++11 auto port](http://clang.llvm.org/extra/clang-modernize.html). Initial requirement: drop XP and support clang/llvm
+* [Clean header include](https://code.google.com/p/include-what-you-use/) 
+* address sanitizer (gcc or clang)
+* valgrind (not sure it can run PCSX2, maybe limit the scope to plugin)
+* reformat the core/plugin to a constant style with tool like astyle
+
+# Very very long term feature
+Those features will require a lots of work! It would require months if not years. There are listed here for completeness ;)
+* PS2 ROM reimplementation (wrongly named HLE bios)
+* Android X86 port
+* Win/Linux ARM port