add a readme about using own compiled OpenModelica

Belonging to [master]:
  - OpenModelica/OMCompiler#2540

README-UseOwnCompiledOpenModelica.txt

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+
+Using your own OpenModelica
+===========================
+Adrian.Pop@liu.se
+2018-06-28
+
+
+Windows:
+========
+You compiled OpenModelica and now you have a directory called build
+inside your OpenModelica cloned repository containing bin, lib and so on.
+To use it you need to set two environment variables:
+OPENMODELICAHOME=C:\Path\To\OpenModelica\build
+OPENMODELICALIBRARY=C:\Path\To\OpenModelica\lib\omlibrary
+NOTE: If you don't set these and have another OpenModelica
+      system installation the dlls and libraries from that
+	  installation will be used and you will have a lot
+	  of issues.
+
+In general I use mingw64 terminal to test omc or OMEdit.
+To set the paths and the environment variables each time I
+open the mingw64 terminal I have in my .bashrc as last line:
+source ~/om.sh
+and my om.sh contains:
+#--- start om.sh
+# set the path to git and svn executables
+export PATH=$PATH:/c/bin/git/bin/:/c/bin/jdk/bin:/c/Program\ Files/TortoiseSVN/bin
+# set the paths to my own compiled OpenModelica build directory
+export OPENMODELICAHOME="c:\\home\\adrpo33\\dev\\OpenModelica\\build"
+export OPENMODELICALIBRARY="c:\\home\\adrpo33\\dev\\OpenModelica\\build\\lib\\omlibrary"
+echo $OPENMODELICAHOME
+echo $OPENMODELICALIBRARY
+echo $PATH
+#--- end om.sh
+
+Then you can start your mingw64 terminal and start omc or OMEdit from
+OpenModelica/build/bin directory.
+
+Linux:
+======
+On Linux we don't need to set any of the OPENMODELICAHOME or OPENMODELICALIBRARY
+as the omc or OMEdit executables know where to use the libraries from via RPATH.
+