doc(CONTRIBUTING): link to docker instructions in wiki (#1185)
doc(CONTRIBUTING): link to docker instructions in wiki (#1185)

Closes #1136.
postgraphql/postgraphql,calebmer/postgraphql,calebmer/postgraphql,calebmer/postgraphql,postgraphql/postgraphql,calebmer/postgraphql,postgraphql/postgraphql,postgraphql/postgraphql