Update README.md

- Make the heading a oneliner by stripping unimportant stuff (this repo is about ST as a whole, not only ST3)
- Update the search link after the recent github changes (https://github.com/blog/1492-repository-search-on-all-repositories)
- Fix a typo
- Do not scare people away by the amount of things they need to consider when creating an issue
- Probably other (un)notable things I've forgotten