Add rider cache directory to Unity ignore list

Riders .idea folder contains a mix of cache, user configuration and settings files.
Similar to Visual Studios .vs folder it should be ignored.
There may be some files (e.g shared dictionaries and settings) in this folders that could be shared.
These can later be manually excluded from the ignore file.

Unity.gitignore

@@ -26,6 +26,9 @@
 # Visual Studio cache directory
 .vs/
 
+# Rider cache directory
+.idea/
+
 # Gradle cache directory
 .gradle/