[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

Unity.gitignore

@@ -1,15 +1,17 @@
-[Ll]ibrary/
-[Tt]emp/
-[Oo]bj/
-[Bb]uild/
-[Bb]uilds/
-[Ll]ogs/
+# This .gitignore file should be placed at the root of your Unity project directory
+
+/[Ll]ibrary/
+/[Tt]emp/
+/[Oo]bj/
+/[Bb]uild/
+/[Bb]uilds/
+/[Ll]ogs/
 
 # Never ignore Asset meta data
-![Aa]ssets/**/*.meta
+!/[Aa]ssets/**/*.meta
 
 # Uncomment this line if you wish to ignore the asset store tools plugin
-# [Aa]ssets/AssetStoreTools*
+# /[Aa]ssets/AssetStoreTools*
 
 # TextMesh Pro files
 [Aa]ssets/TextMesh*Pro/