modifies lib exclude, adds example lib include

explicitly excludes the `_Libraries` directory, and provides example of
adding sub directories back in. Should be used for including custom and
third party (non -Beckhoff shipped) libraries, improving maintainability

TwinCAT3.gitignore

@@ -21,5 +21,11 @@ LineIDs.dbg
 LineIDs.dbg.bak
 _Boot/
 _CompileInfo/
-_Libraries/
-_ModuleInstall/
+_ModuleInstall/
+
+# Exclude all libraries
+/**/_Libraries/*/
+
+# To include a specific library directory (i.e. third party/custom libs), 
+# use pattern `!/**/_Libraries/<directory name>/` i.e. `!/**/_Libraries/www.tcunit.org/`
+#