[Documentation] Document Object ManagerSniff

Describe why the use of Object Manager is considered as a bad practice.

### Acceptance criteria
1. `ObjectManagerSniff.md` file is created.
2. Explanation about why the use of Object Manager is discouraged.
3. _Good_ and _bad_ code snippets provided.

[ExtDN's TemplateObjectManagerSniff.md](https://github.com/extdn/extdn-phpcs/blob/master/Extdn/Sniffs/Templates/TemplateObjectManagerSniff.md) can be used as a good example.

Start by reviewing ExtDN's TemplateObjectManagerSniff.md, linked in the issue, for structure and level of explanation. Create ObjectManagerSniff.md with why Object Manager is discouraged and include both good and bad code snippets. Done means the file exists and all three acceptance criteria are covered.

Written by the indexing model from the issue text.