Getting Started guide missing WHITELISTED_OPENRA_ASSEMBLIES

[pchote]

The WHITELISTED_OPENRA_ASSEMBLIES list does not include OpenRA.Mods.Cnc.dll - novices coming from TD/RA-based projects might be overlooking it at first sight. This probably should be mentioned that it needs to be listed in such scenarios.

Originally posted by @GraionDilach in #117 (comment)

[Mailaender]

As make check will tell you these are missing I suggest adding that to the Travis file.

[pchote]

Modders should be running make check locally too, so Getting Started is the correct place.