Add CONTRIBUTING.md
Add CONTRIBUTING.md

Based on other project languages, but newly written. Typos and grammer
mistakes ahead!

nmaier/about-addons-memory,nmaier/samchanedit,nmaier/samchanedit,nmaier/about-addons-memory,nmaier/FastPrevNext,nmaier/extsdk,nmaier/NoPluginBar,nmaier/NoPluginBar,nmaier/extsdk,nmaier/FastPrevNext,nmaier/repagination,nmaier/repagination,nmaier/about-addons-memory