Add changelog for 1.3.0 release

CHANGELOG-1.3.md

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+CHANGELOG for 1.3.x
+===================
+
+This changelog references the relevant changes (bug and security fixes) done
+in 1.3.x patch versions.
+
+1.3.1 (????-??-??)
+------------------

UPGRADE-1.3.md

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+UPGRADE from 1.2 to 1.3
+=======================
+
+Pull requests completed for the 1.3.0 release:
+
+ * [#227](https://github.com/doctrine/mongodb/pull/227): Specify time limit operation on a mongodb cursor
+   * Adds `maxTimeMS()` method to query builder and cursor
+ * [#233](https://github.com/doctrine/mongodb/pull/233): Allow Event Listeners the ability to modify context information in the event
+   * Allows changes to the options for the following events: preAggregate, preBatchInsert, preDistinct, preFind, preFindAndRemove, preFindAndUpdate, preFindOne, preGetDBRef, preGroup, preInsert, preMapReduce, preNear, preRemove, postRemove, preSave, preUpdate, postUpdate
+ * [#234](https://github.com/doctrine/mongodb/pull/234): Add support for `$comment` operator
+   * Adds `comment()` method to query builder
+ * [#235](https://github.com/doctrine/mongodb/pull/235): Add support for `$setOnInsert` operator
+   * Adds `setOnInsert()` method to query builder
+ * [#238](https://github.com/doctrine/mongodb/pull/238): Bump PHP and mongo version requirements
+ * [#240](https://github.com/doctrine/mongodb/pull/240): Add new MongoDB 3.2 features to aggregation builder
+   * Adds `sample()`, `indexStats()` and `lookup()` methods to aggregation builder
+   * Adds `avg()`, `max()`, `min()`, `stdDevPop()`, `stdDevSamp()`, `sum()` methods to project stage
+   * Adds `minDistance()` method to geoNear stage
+   * Adds `includeArrayIndex()` and `preserveNullAndEmptyArrays()` methods to unwind stage
+ * [#241](https://github.com/doctrine/mongodb/pull/241): Add query operators introduced with MongoDB 3.2
+   * Adds `bitsAllClear()`, `bitsAllSet()`, `bitsAnyClear()`, `bitsAnySet()`, `caseSensitive()`, `diacriticSensitive()` methods to query builder
+ * [#251](https://github.com/doctrine/mongodb/pull/251): Corrected fluent interface docblocks
+ * [#255](https://github.com/doctrine/mongodb/pull/255): Add expr method to aggregation expression object
+   * Adds `expr()` method to the aggregation Expr class
+ * [#256](https://github.com/doctrine/mongodb/pull/256): Allow using operators in group stages
+   * Adds all methods from the `Operator` class to the group stage