Update README about support for template variables

[vschettino]

As the README says:

We have aggressive caching enabled due to GitHub's rate limiting policies. When selecting a time range like "Last hour", a combination of the queries for each panel and the time range is cached temporarily.

• I am trying to use a template variable in the "Query" field and it's not working

Template variables are currently not supported outside of the "Owner / Organization" and "Repository" fields.

I see it's done (#14, #29), but I feel like the FAQ on the README is outdated. I just tested and it works fine having a "milestone" variable addressed on Query field, as the prints show:

If you guys think this is a good idea, I can open a PR. We could just remove this FAQ item and add extra info about the support in the "variables" section.

[kminehart]

Yeah good call. This was fixed in this PR: #42.

I still want to clean up the way variables are handled (#43) but it definitely works now :D