Add contribution guide

CONTRIBUTING.md

+Install using
+
+`Pkg.clone("https://gitlab.control.lth.se/cont-frb/SystemIdentification.git")`
+
+Test using 
+
+`Pkg.test("SystemIdentification")`
+
+
+Commit your updates using the guide at
+http://julia.readthedocs.org/en/latest/manual/packages/#package-development
+
+`cd(Pkg.dir("SystemIdentification"))`
+
+`"Git commands..."`
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