Added release notes for v1.3.1.

Author: seidewitz

org.modeldriven.fuml/doc/release-notes/fuml-1.3.1.md

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+### fUML Reference Implementation
+### v1.3.1 Release Notes
+
+This is a production release of the fUML Reference Implementation, tracking the issue resolutions approved by 
+the fUML 1.4 Revision Task Force (RTF).
+
+**IMPORTANT NOTE**
+
+The fUML 1.4 RTF was unusual, in that it's primary task was to migrate fUML to UML 2.5.1. Therefore, fUML 1.4 
+will have no functional differences from fUML 1.3, but the structure of the fUML syntax and semantics has been
+significantly re-organized to follow the new abstract syntax package structure introduced in UML 2.5.
+The Java package hierarchy for the syntax and semantics implementation classes has been correspondingly updated
+in the Reference Implementation. In addition, while the Java package names are still based on the names of the 
+corresponding UML packages, they are now rendered entirely in lower case, per usual Java convention (e.g., 
+`fuml.semantics.actions` rather than `fUML.Semantics.Actions`).
+
+### fUML Execution Engine
+
+The current base release of the fUML Execution Engine is v1.3.0 (see the [v1.3.0 Release Notes](./fUML-1.3.0.md)), 
+conforming to v1.3.0 of the [fUML Specification](http://www.omg.org/spec/FUML/1.3.0).
+The v1.3.1 tracking release includes implementation of resolutions to the following OMG issues, which have
+been approved by the fUML Revision Task Force for fUML 1.4 but have not yet been
+formally adopted.
+
+The full list of open fUML issues is available at http://issues.omg.org/issues/task-force/FUML14.
+
+The following issues were resolved by the fUML 1.4 RTF:
+
+[FUML 14-9](http://issues.omg.org/issues/task-force/FUML14#issue-42353) Migrate fUML to UML 2.5.1
+
+[FUML 14-10](http://issues.omg.org/issues/task-force/FUML14#issue-42655) The ExecutionFactory class descriptions need 
+to be updated in the specification for CentralBufferNode and DataStoreNode
+
+[FUML 14-21](http://issues.omg.org/issues/task-force/FUML14#issue-42985) Correction to the resolution to issue 14-10
+
+### XMI Loader/Infrastructure
+
+In order to implement the migration to UML 2.5.1, as required by the resolution to OMG Issue FUML 14-9, 
+this tracking release also updates the XMI Loader/Infrastructure for the new structure of the UML 2.5.1 metamodel.
+
+#### Bugs
+
+None
+
+#### Improvements
+
+[Issue #31](https://github.com/ModelDriven/fUML-Reference-Implementation/issues/31) Update the XMI Loader implementation for the new structure of UML 2.5.1
+