Add CONTRIBUTE file. [doc]

CONTRIBUTE.md

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+# CONTRIBUTING TO FACETS
+
+* Facets methods must have no external dependencies. (The only possible
+  exception to this is fi we decide to spin functor methods off to a
+  separate functor gem, but for now that is not happening.)
+
+* Methods must be tested either via a Lemon unit test or a QED demo,
+  preferrably both. The Lemon unit tests are for testing a method in
+  detail whereas the QED demos are for demonstrating usage.
+
+* Facets is divided into two parts, *core* and *standard* libraries.
+  All of the core library is loaded by default when using `require 'facets'`
+  (actually, almost all). The standard library (sometimes called the *more*
+  library) must be required per-script.
+
+* Some core methods are included on a *trial* basis, and these are not 
+  necessary loaded automatically with `require 'facets'`.
+