Add note about unsupported YAML features in gemfile: line in default.yml (#865)

Related to #863 and [this
comment](#862 (comment)):

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

Author: pilaf

config/default.yml

@@ -32,6 +32,10 @@
 # your repository, and then set the `gemfile` option below to the name you gave
 # the file.
 # (Generate lock file by running `bundle install --gemfile=.overcommit_gems.rb`)
+#
+# NOTE: the following line will be parsed by a regexp rather than a proper YAML
+# parser, so avoid any values other than false or a string, and don't use inline
+# comments
 gemfile: false
 
 # Where to store hook plugins specific to a repository. These are loaded in