New README file with multicore specific information.

The old readme file is now `README.stock.adoc`.

README.md

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+# Multicore OCaml
+
+A concurrent and shared-memory parallel extension of the OCaml compiler.
+
+Multicore OCaml cleanly separates abstractions for concurrency (overlapped
+execution) from parallelism (simultaneous execution). Concurrency is expressed
+through [effect handlers](http://kcsrk.info/papers/system_effects_feb_18.pdf)
+and parallelism through
+[domains](https://github.com/ocaml-multicore/ocaml-multicore/blob/master/stdlib/domain.mli).
+Much of the work in supporting shared memory parallelism is the development of a
+mostly-concurrent, generational, mark-and-sweep collector that strikes a balance
+between single-threaded performance and feature backwards compatibility, and
+multicore scalability.
+
+See the
+[wiki](https://github.com/ocaml-multicore/ocaml-multicore/wiki) for more
+resources.
+
+## Variants
+
+* Parallel Minor GC (default) -- A variant that performs stop-the-world parallel
+  minor collection. The branch is `parallel_minor_gc`.
+* Concurrent Minor GC -- A variant that garbage collects each domain's minor
+  heap concurrently. The branch is `master`.
+
+## Notes
+
+The original README.adoc file has been moved to
+[README.stock.adoc](README.stock.adoc).