Added release notes for v1.2.3.

Author: seidewitz

org.modeldriven.fuml/doc/release-notes/fuml-1.2.3.md

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+### fUML Reference Implementation
+### v1.2.3 Release Notes
+
+This is a production release of the fUML Reference Implementation tracking the issue resolutions approved in Ballots 3 and 4 of the fUML 1.3 Revision Task Force.
+
+### fUML Execution Engine
+
+The current base release for the fUML Execution Engine is v1.2.0b (see the [v1.2.0b Release Notes](./fuml-1.2.0b.md)), conforming to v1.2.1 of the [fUML Specification](http://www.omg.org/spec/FUML/1.2.1). The v1.2.3 tracking release includes implementation of resolutions to the following OMG issues, which have been approved by the fUML Revision Task Force for fUML 1.3 but have not yet been formally adopted, in addition to resolutions already implemented in [v1.2.2](./fuml-1.2.2.md). 
+
+The full list of open fUML issues is available at http://issues.omg.org/issues/task-force/FUML13.
+
+[FUML 13-1](http://issues.omg.org/issues/task-force/FUML13#issue-38293) SendSignalAction completion semantics
+
+[FUML 13-4](http://issues.omg.org/issues/task-force/FUML13#issue-38296) The fUML subset should include central buffer nodes and data stores
+
+[FUML 13-16](http://issues.omg.org/issues/task-force/FUML13#issue-38600) Introduce CallEvent, AcceptCallAction and ReplyAction to keep consistency with PSSM
+
+[FUML 13-49](http://issues.omg.org/issues/task-force/FUML13#issue-40992) Correction to the resolution to issue [FUML 13-25](http://issues.omg.org/issues/task-force/FUML13#issue-40627)
+
+[FUML 13-60](http://issues.omg.org/issues/task-force/FUML13#issue-41143) Simplification to the resolution to issue [FUML 13-1](http://issues.omg.org/issues/task-force/FUML13#issue-38293)
+
+### XMI Loader/Infrastructure
+
+This tracking release includes all resolutions from [v1.2.0b](./fuml-1.2.0b.md) and previous maintenance releases.
+
+#### Bug Fixes
+
+None
+
+#### Improvements
+
+None