Guidelines for Adblock Filter Lists

I think it is worth considering compiling a guideline for Adblock Filter Lists.

In my opinion, there are two main groups of policies:
- Syntactic / formatting specifications
- Application specifications: routines, best practices <-> bad practices

I think such guidelines could be:
- Preferred scriptlet quote marks,
- Formatting of CSS selectors, e.g. whether there should be a space before `!important`
- Application of general <-> specific principles (e.g. EasyList is a very general list, and AG filters are more specific),
- Contraindication of multiple selectors within a rule: `example.com##.ad1, .ad2`
- Taking into account the limitations of content blockers, e.g. creating regular exception network rules next to scriptlets
- Comment guides, e.g. put one space after comment marker, link issues before rules
- Rule ordering/grouping, where to place a new rule
- Where to place a new domain if the rule is already exists
- etc.

The description of the filter repo doesn't cover such things:
https://github.com/AdguardTeam/AdguardFilters/blob/master/CONTRIBUTING.md

I would be happy if you could share your opinion. 🙂

@ameshkov @Alex-302 @zloyden @AdamWr