[CHANGE #405] adapted link to moved wiki

__README

-See http://www.openwalnut.org/projects/openwalnut/wiki/Module_ExternalDevelopment for an extensive introduction to external (out-of-source) module development.
+See https://bsvgit.informatik.uni-leipzig.de/openwalnut/openwalnut/wikis/Module_ExternalDevelopment for an extensive introduction to external (out-of-source) module development.
 
 Building all toolboxes
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