Add Contributing to Skia section of docs

site/dev/contrib/flatten.md

+Flattenables
+============
+
+Many objects in Skia, such as SkShaders and other effects on SkPaint, need to be flattened into a data stream for either transport
+or as part of the key to the font cache. Classes for these objects should derive from SkFlattenable or one of its subclasses. If you
+create a new flattenable class, you need to make sure you do a few things so that it will work on all platforms:
+
+1: Override the method flatten (the default scope is protected):
+
+~~~~
+virtual void flatten(SkFlattenableWriteBuffer& buffer) const SK_OVERRIDE {
+    this->INHERITED::flatten(buffer);
+    // Write any private data that needs to be stored to recreate this object
+}
+~~~~
+
+2: Override the (protected) constructor that creates an object from an SkFlattenableReadBuffer:
+
+~~~~
+SkNewClass(SkFlattenableReadBuffer& buffer)
+: INHERITED(buffer) {
+    // Read the data from the buffer in the same order as it was written to the
+    // SkFlattenableWriteBuffer and construct the new object
+}
+~~~~
+
+3: Declare a set of deserialization procs for your object in the class declaration:
+We have a macro for this:
+
+~~~~
+public:
+
+SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkNewClass)
+~~~~
+
+4: If your class is declared in a .cpp file or in a private header file, create a function
+to register its group:
+This occurs in cases where the classes are hidden behind a factory, like many effects and shaders are.  Then in the parent
+class header file (such as SkGradientShader) you need to add:
+
+~~~~
+public:
+
+SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
+
+
+Then in the cpp file you define all the members of the group together:
+SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkGroupClass)
+
+    SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkMemberClass1)
+
+    SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkMemberClass2)
+
+    // etc
+
+SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
+~~~~
+
+
+5: Register your flattenable with the global registrar:
+You need to add one line to SkFlattenable::InitalizeFlattenables(). To register the flattenable in a Skia build, that function is
+defined in SkGlobalInitialization_default.cpp. For Chromium, it is in SkGlobalInitialization_chromium.cpp.
+For a single flattenable add
+
+~~~~
+SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkNewClass)
+~~~~
+
+For a group, add
+
+~~~~
+SkGroupClass::InitializeFlattenables();
+~~~~
+