README: Replace stale environment variable reference
README: Replace stale environment variable reference

Catching up with 6be4c253 (*: remove support for environment
variables, 2018-12-10, #861).

Also link to the troubleshooting docs, since the easier it is to
discover those, the fewer bug reports we'll get with insufficient
detail ;).

derekhiggins/installer,derekhiggins/installer