ObjectiveC: add objective C code guidelines

Useful links:

http://google.github.io/styleguide/objcguide.html
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.html

Structure:

* Introduction
* Naming Conventions
* Comments and Documentation (you can get it from here: https://github.com/AdguardTeam/CodeGuidelines/blob/master/Java.md#comments-and-documentation)
* General Code Quality (short language-specific points about code quality)
* Best Practices (detailed guides, more common than the code quality points)