Add a readme

Author: Johan Tibell

README.md

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+## unordered-containers
+
+Efficient hashing-based container types.  The containers have been optimized for
+performance critical use, both in terms of large data quantities and high speed.
+
+The declared cost of each operation is either worst-case or amortized, but
+remains valid even if structures are shared.
+  
+For developer and contributor documentation see docs/developer-guide.md.

docs/developer-guide.md

@@ -110,3 +110,11 @@ updating the spine of the tree).
 `Full` is just an optimized version of `BitmapIndexed` that allows us faster
 indexing, faster copying on modification (given that its size is statically
 know), and lower memory use.
+
+## Strictness
+
+Everything is strict. Laziness is the enemy of predictable performance.
+
+## Backwards compatibility policy
+
+We support the last 3 major GHC releases.