[issue-linker] Onboard users to add access tokens

[mcomella]

e.g. warn them when they hit the rate limit.

[mcomella]

We'll want:

• Docs on how to do it
• Link to ^ from readme, feature
• Display link to docs from app (easy option: store "rate limited" state in DB and, if rate limited, display next to "how?"; this won't work for issues list, however)

[mcomella]

Here's the official docs on getting a personal access token: https://help.github.com/articles/creating-a-personal-access-token-for-the-command-line/

[mcomella]

I did the easy option: I wrote the docs and added an always-on tip to add an access token.