add README

README.md

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+`wterl` is an Erlang interface to the WiredTiger database, and is written
+to support a Riak storage backend that uses WiredTiger.
+
+This backend currently supports only key-value storage and retrieval.
+
+Remaining work includes:
+
+* The `wterl:session_create` function currently returns an error under
+  certain circumstances, so we currently ignore its return value.
+* The `riak_kv_wterl_backend` module is currently designed to rely on the
+  fact that it runs in just a single Erlang scheduler thread, which is
+  necessary because WiredTiger doesn't allow a session to be used
+  concurrently by different threads. If the KV node design ever changes to
+  involve concurrency across scheduler threads, this current design will no
+  longer work correctly.
+* Currently the `riak_kv_wterl_backend` module is stored in this
+  repository, but it really belongs in the `riak_kv` repository.
+* There are currently some stability issues with WiredTiger that can
+  sometimes cause errors when restarting KV nodes with non-empty WiredTiger
+  storage.
+
+Future support for secondary indexes requires WiredTiger features that are
+under development but are not yet available.
+
+Deploying
+---------
+
+You can deploy `wterl` into a Riak devrel cluster using the `enable-wterl`
+script. Clone the `riak` repo, change your working directory to it, and
+then execute the `enable-wterl` script. It adds `wterl` as a dependency,
+runs `make all devrel`, and then modifies the configuration settings of the
+resulting dev nodes to use the WiredTiger storage backend.