Add note about unsupported YAML features in gemfile: line in default.yml

[pilaf]

Related to #863 and this comment:

Having a similar problem since we had this line in our .overcommit.yml:

gemfile: Gemfile # enforce bundled version of overcommit

And now overcommit doesn't strip out the inline comment, resulting in this weird looking error message:

Problem loading 'Gemfile # enforce bundled version of overcommit': /path/to/project/Gemfile # enforce bundled version of overcommit not found 

I think adding support for comments in the gemfile: regexp is likely overkill and may still not be enough when the next person tries to use yet another YAML feature in that line, but perhaps this little warning would help someone else avoid tripping.

[sds]

Thank you! Appreciate it.