Add changelog for 1.2

Author: alcaeus

CHANGELOG-1.2.md

@@ -0,0 +1,22 @@
+CHANGELOG for 1.2.x
+===================
+
+This changelog references the relevant changes done in 1.2 patch versions.
+
+1.2.0 (2017-10-24)
+------------------
+
+All issues and pull requests in this release may be found under the
+[1.2 milestone](https://github.com/doctrine/mongodb-odm/issues?q=milestone%3A1.2).
+
+* [#1448](https://github.com/doctrine/mongodb-odm/pull/1448) adds a builder for aggregation pipeline queries, similar to the query builder.
+* [#1513](https://github.com/doctrine/mongodb-odm/pull/1513) adds a trait to avoid re-implementing `closureToPhp` in custom type classes.
+* [#1518](https://github.com/doctrine/mongodb-odm/pull/1518) adds `updateOne` and `updateMany` methods to the query builder. 
+* [#1519](https://github.com/doctrine/mongodb-odm/pull/1519) adds a command to validate mapping.
+* [#1577](https://github.com/doctrine/mongodb-odm/pull/1577) allows priming fields in inverse references without specifying a repositoryMethod.
+* [#1600](https://github.com/doctrine/mongodb-odm/pull/1600) adds an `AbstractRepositoryFactory` as base class when creating an own repository factory.
+* [#1612](https://github.com/doctrine/mongodb-odm/pull/1612) adds support for immutable documents via the `readOnly` mapping option.
+* [#1620](https://github.com/doctrine/mongodb-odm/pull/1620) adds support for specifying `readPreference` on a document level, replacing `slaveOkay`.
+* [#1623](https://github.com/doctrine/mongodb-odm/pull/1623) adds a generic reference object as successor to `dbRef`.
+* [#1654](https://github.com/doctrine/mongodb-odm/pull/1654) adds support for aggregation pipeline stages added in MongoDB 3.4.
+* [#1661](https://github.com/doctrine/mongodb-odm/pull/1661) allows specifying a custom starting ID for `IncrementGenerator`.