Psa Extern Development Guide#779

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derek-so wants to merge 5 commits intop4lang:mainfrom

derek-so:psa_extern_doc

Commits on May 16, 2019

adding docs for how to develop externs in psa switch
Derek So
committed
added helpful background knowledge
Derek So
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grammar fixes
Derek So
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added common issues
Derek So
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grammar and spelling
Derek So
committed