document rustpkg use with make

[metajack]

Using make and rustpkg together is a bit tricky, mostly because the output files don't have known names (they contain hashes).

A combination of wildcard targets and order-only prerequisites is needed to make this work, and should be documented.

[catamorphism]

Details: http://irclog.gr/#browse/irc.mozilla.org/servo/34000 and http://irclog.gr/#browse/irc.mozilla.org/servo/34029

[pnkfelix]

rustpkg was removed in commit 25fe2ca. Closing.