Add a guide about diagnosing Hot Code Push issues

[namenotrequired]

I spent many days researching why Hot Code Push was working unreliably in an app of mine. After I solved it, I saw some other people comment on similar issues in the Meteor slack. So I decided to write down what I had learned. @filipenevola suggested publishing it in the guide.

Looking forward to your suggestions.

TODO:

• [x] If this is a significant change, update CHANGELOG.md
• [x] Use <h2 id="foo"> instead of ## Foo for headers
• [x] Leave a blank line after each header

[CLAassistant]

All committers have signed the CLA.