Description how to override message service library added (#205)

* Description how to override message service library added

Fixes #204

The new MD file describes how to override message service library
included in generate service. This can be useful for many reasons, e.g.
custom library should be used, etc.

* Link to JitPack added to documention.

* JitPack URL fixed.

* -Djava.ext.dirs replaced by -classpath 

Option -Djava.ext.dirs has been deprecated in Java 8u40.

* Link to example in eiffel-remrem-publish added.

Author: z-sztrom

README.md

@@ -42,6 +42,8 @@ Eiffel REMReM Generate takes a partial message in JSON format, validates it and
 
 7. [**Logging**](wiki/markdown/logging.md)
 
+7. [**Overriding Message Service Library**](wiki/markdown/overriding.md)
+
 # About this repository
 The contents of this repository are licensed under the [Apache License 2.0](./LICENSE).
 

wiki/markdown/overriding.md

@@ -0,0 +1,63 @@
+## Overriding Message Service Library
+
+When building ``eiffel-remrem-generate`` and ``eiffel-remrem-publish`` services,
+target packages include also ``eiffel-remrem-semantics`` library
+of a version given by services' ``POM`` file. This can be easily verified sending
+``/versions`` request to the services. It produces something like that
+~~~
+{
+  "serviceVersion": {
+    "version": "2.1.4"
+  },
+  "endpointVersions": {
+    "semanticsVersion": "2.2.1"
+  }
+}
+~~~
+
+If there is a need, for whatever reason, to use different version of semantics
+library, appropriate JAR file can be stored in a directory listed as value of Java
+option ``-classpath`` utilizing [wildcard mechanism](https://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html).
+For example, when ``eiffel-remrem-semantics-2.1.3.jar``
+is stored in, say, ``/eiffel/protocols``, the directory must be included in ``java``
+command
+~~~
+java ... -classpath "...:/eiffel/protocols/*:..." -jar .../generate.war
+~~~
+
+Note, that [Resources 'earlier' on the classpath take precedence over resources that are specified after them](https://stackoverflow.com/questions/6644440/java-which-of-multiple-resources-on-classpath-jvm-takes).
+
+New request to ``/versions`` now clearly proves that implementation from newly
+deployed library is used instead of build-in one.
+~~~
+{
+  "serviceVersion": {
+    "version": "2.1.4"
+  },
+  "endpointVersions": {
+    "semanticsVersion": "2.1.3"
+  }
+}
+~~~
+
+
+This  feature can be utilized to load completely different implementation of Eiffel
+protocol, for example ``eiffel3``, by adding corresponding library into directory
+included in ``-classpath``, i.e. ``/eiffel/protocols``. The newly added library
+causes that output of ``/versions`` request looks like this
+~~~
+{
+  "serviceVersion": {
+    "version": "2.1.4"
+  },
+  "endpointVersions": {
+    "semanticsVersion": "2.1.3",
+    "eiffel3MessagingVersion": "29.0.9"
+  }
+}
+~~~
+
+Particular version of ``eiffel-remrem-semantics`` can be downloaded from
+[JitPack](https://jitpack.io/#eiffel-community/eiffel-remrem-semantics).
+
+A practial example can be found in chapter [Override RemRem-Publish Eiffel Protocol Version](https://github.com/eiffel-community/eiffel-remrem-publish/blob/master/wiki/markdown/run.md#override-remrem-publish-eiffel-protocol-version).