Add Github to the list of supported social providers in documentation #240

Job	Run time
Coding Standard & Static Analysis	32s
testsuite (8.1, pgsql)	25s
testsuite (8.1, mysql)	33s
testsuite (8.2, mysql)	35s
testsuite (8.2, sqlite)	27s
testsuite (8.1, sqlite)	21s
testsuite (8.3, sqlite)	21s
testsuite (8.3, pgsql)	25s
testsuite (8.2, pgsql)	28s
testsuite (8.4, mysql)	25s
testsuite (8.4, sqlite)	27s
testsuite (8.3, mysql)	25s
testsuite (8.4, pgsql)	39s
	6m 3s

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