Add 2.1.9 release notes

Added 2.1.9 release notes.

Documentation/Release-Notes/MaxScale-2.1.9-Release-Notes.md

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+# MariaDB MaxScale 2.1.9 Release Notes
+
+Release 2.1.9 is a GA release.
+
+This document describes the changes in release 2.1.9, when compared to release
+[2.1.8](MaxScale-2.1.8-Release-Notes.md).
+
+If you are upgrading from release 2.0, please also read the following release
+notes:
+
+* [2.1.8](./MaxScale-2.1.8-Release-Notes.md)
+* [2.1.7](./MaxScale-2.1.7-Release-Notes.md)
+* [2.1.6](./MaxScale-2.1.6-Release-Notes.md)
+* [2.1.5](./MaxScale-2.1.5-Release-Notes.md)
+* [2.1.4](./MaxScale-2.1.4-Release-Notes.md)
+* [2.1.3](./MaxScale-2.1.3-Release-Notes.md)
+* [2.1.2](./MaxScale-2.1.2-Release-Notes.md)
+* [2.1.1](./MaxScale-2.1.1-Release-Notes.md)
+* [2.1.0](./MaxScale-2.1.0-Release-Notes.md)
+
+For any problems you encounter, please consider submitting a bug report at
+[Jira](https://jira.mariadb.org).
+
+## Bug fixes
+
+[Here is a list of bugs fixed in MaxScale 2.1.9.](https://jira.mariadb.org/issues/?jql=project%20%3D%20MXS%20AND%20issuetype%20%3D%20Bug%20AND%20status%20%3D%20Closed%20AND%20fixVersion%20%3D%202.1.9)
+
+* [MXS-1377](https://jira.mariadb.org/browse/MXS-1377) maxscale doesn't cleanup pid file on startup error
+* [MXS-1295](https://jira.mariadb.org/browse/MXS-1295) MaxScale's readwritesplit router does not take into account the fact that stored procedure call may change the value of  a user variable
+
+## Packaging
+
+RPM and Debian packages are provided for the Linux distributions supported by
+MariaDB Enterprise.
+
+Packages can be downloaded [here](https://mariadb.com/resources/downloads).
+
+## Source Code
+
+The source code of MaxScale is tagged at GitHub with a tag, which is identical
+with the version of MaxScale. For instance, the tag of version X.Y.Z of MaxScale
+is maxscale-X.Y.Z.
+
+The source code is available [here](https://github.com/mariadb-corporation/MaxScale).