update gitignore to ignore all backup and upgrade files

.gitignore

@@ -71,3 +71,16 @@ crashlytics-build.properties
 /[Aa]ssets/[Ss]treamingAssets/aa.meta
 /[Aa]ssets/[Ss]treamingAssets/aa/*
 
+# Backup & report files from converting an old project file
+# to a newer Visual Studio version. Backup files are not needed,
+# because we have git ;-)
+_UpgradeReport_Files/
+Backup*/
+UpgradeLog*.XML
+UpgradeLog*.htm
+*.bak
+
+# JetBrains Rider
+.idea/
+*.sln.iml
+