Improve documentation for what will be accepted into the database, and other best practices

Many times I have had pull requests denied due reasons not documented in this repo. This is not so much an issue for me, other than being a slight annoyance, but more-so for the AdGuard employees who continuously have to request corrections for research and information I could have provided if better documented.

Due to the lack of documentation, my understanding of this repo has been to add active companies and/or active trackers I find in my query logs that are not already in the companiesdb database. This has proven not to be the case (see the rejected pr's).

It would be good if this was avoidable by clearly documenting what is acceptable and what is not acceptable when adding new companies and trackers.

Below is a non-exhaustive list of issues I have run into when awaiting pull request approval:

- The tracker added is not popular enough (there is no popularity standard),
- The company added is not popular enough (there is no popularity standard),
- The company added did not have a tracker, so was not considered to acceptable to be added to the database as a company even though it was large enough (is being a company not enough to be added to the database? Does it need to have active trackers too? https://github.com/AdguardTeam/companiesdb/pull/217)
- Confusion around whether to add the parent company, or the company in question (An example of this is [Foxtel](https://github.com/AdguardTeam/companiesdb/pull/224))