FreeBSD/doc a25cc9c (r50682)en_US.ISO8859-1/books/porters-handbook/makefiles chapter.xml, share/xml man-refs.ent

Add a USE_GITHUB example on how to use git describe.

Reviewed by:    bcr
Sponsored by:   Absolight
Differential Revision:  https://reviews.freebsd.org/D12047
DeltaFile
+80-0en_US.ISO8859-1/books/porters-handbook/makefiles/chapter.xml
+1-0share/xml/man-refs.ent
+81-02 files

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