remove MStore from master branch for now

We first have to develop the Store better before building
stuff on top of it. MStore was rather in the way of doing
that.

Before ripping MStore out, I made a branch "mstore" with
the current state (as in master branch before this commit),
so it can be developed there.

README.rst

@@ -1,8 +1,7 @@
 BorgStore
 =========
 
-A key/value store implementation in Python, supporting multiple backends,
-data redundancy and distribution.
+A key/value store implementation in Python, supporting multiple backends.
 
 Keys
 ----
@@ -103,15 +102,6 @@ Currently, these storage backends are implemented:
 - Rclone - access any of the 100s of cloud providers [rclone](https://rclone.org/) supports
 - (more might come in future)
 
-MStore
-------
-
-API of MStore is very similar to Store, but instead of directly using one backend
-only (like Store does), it uses multiple Stores internally to implement:
-
-- redundancy (keep same data at multiple places)
-- distribution (keep different data at multiple places)
-
 Scalability
 -----------