Prevent Unity .gitignore from ignoring non-Unity project files

Unity only generates project files in the top-level directory.
However, the current .gitignore will ignore all project files even
if they were not generated by Unity.

It is sometimes necessary to include non-Unity project files for
level editors or other utility projects that have been added to the
repository.

Unity.gitignore

@@ -3,8 +3,8 @@
 [Oo]bj/
 
 # Autogenerated VS/MD solution and project files
-*.csproj
-*.unityproj
-*.sln
-*.suo
-*.user
+/*.csproj
+/*.unityproj
+/*.sln
+/*.suo
+/*.user