Adding build tag documentation

[mzimmerman]

No description provided.

[gen2brain]

In cases where older gcc is used (like in travis ci), and with -tags gcc it will return same/similar error.
Better to just add to docs somethink like "if you get this error... add -tags gcc7"

[mzimmerman]

Makes sense, see if this is better, thanks!

[gen2brain]

Thanks, added.