add instructions for setting GUILE env to README

[charsi]

[OrangeShark]

Thank you for your contribution. What you describe isn't what you reported in #13 where it can't find the guild binary. I think you should mention the error that you got in #13 and then provide the way to fix it like you mentioned here. The environment variables used in the configure script are only visible within the script, so echo $GUILD or echo $GUILE would usually always be empty so it not a valid way to check if you have this issue. So if you fix this I will merge this pull request. If what I described isn't clear, please reply back.

[charsi]

I've changed the wording and created a new pull request.