Added a comment why MemoryCaptures get excluded (#3276)

Unity.gitignore

@@ -8,6 +8,9 @@
 /[Bb]uild/
 /[Bb]uilds/
 /[Ll]ogs/
+
+# MemoryCaptures can get excessive in size.
+# They also could contain extremely sensitive data
 /[Mm]emoryCaptures/
 
 # Asset meta data should only be ignored when the corresponding asset is also ignored