Install manpage when building from source

[yetyetanotherusername]

Building from source as documented in the readme currently leads to an installation without the manpage. Would it be ok to add -Demit-docs argument so that new users don't end up with no manpage?

If yes, perhaps also pandoc should be added to the build requirements, as otherwise the build fails.

[mitchellh]

emit-docs is already true if pandoc is available. We should probably document this for the packaging directions though.

[mitchellh]

#2726