[Unity] Added leading slashes to ignored directories so that valid...

[Unity] Added leading slashes to ignored directories so that valid subdirectories aren't ignored incorrectly (#2980)

* Added leading slashes to ignored directories so that valid subdirectories aren't ignored incorrectly

* Added comment to recommend .gitignore placement; added leading slash for AssetStoreTools rule

* Added a leading slash to never ignore .meta files in the root Asset folder