Document some trouble shooting steps and probot-stale behavior

[rallytime]

After configuring the probot-stale integration, I learned several things that weren't documented very well, or were difficult to find. Documentation should be added to the README to help people troubleshoot:

• The stale probot runs every hour.
• When it runs each hour, the time it starts is randomly determined.
• "stale" issues doesn't mean just when a user comments on an issue. It can also mean when a milestone is applied. It has to do with the updated search item.
• When the bot is first started (or the integration is installed, depending on your setup method), and the config file is already in place, the bot will not comment on things right away. However, if the config file is pushed up after the bot starts, it will start searching for issues immediately. This is true for configuration file changes as well.

This issue is similar to the request/discussion in #34, but includes a couple of other things I found helpful once I understood how everything worked. This is also related to the questions posed in #33.

[rallytime]

I'll try to get some of these items added to the README file soon.

[stkent]

All great additions! Is there a way to force the first round of commenting to happen immediately without pushing up a new config? Would be useful for development :)

[rallytime]

@stkent Not that I know of.

@bkeepers Is that something that could be added with flag or something?

[rallytime]

Ok, I added documentation for these items in PR #41.