Update README.md with information about NOT_REQUESTED & DENIED_ALWAYS statuses for Android

[sebj54]

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [x] Documentation changes
• [ ] Other... Please describe:

What is the purpose of this PR?

I had an issue with this plugin. I always got DENIED_ALWAYS after reinstalling the app and couldn't find why… So I dug in the issues and find one very useful. It could be useful to others so I added extra documentation.

Issue related: https://github.com/dpa99c/cordova-diagnostic-plugin/issues/191#issuecomment-291467086

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No