Updates .gitattributes, added export-ignore.

Excludes `user_guide_src` via `--prefer-dist` composer installation. 

Refer to this #5843 (comment) based on this #3831

> This basically just removes a number of development-only files (like the user guide source and our tests) from any installs made through composer with the --prefer-dist flag set.
> 
> Helps create a faster install with less unnecessary files to new projects.

.gitattributes

@@ -18,8 +18,6 @@ readme.rst
 tests/codeigniter/ export-ignore
 tests/travis/ export-ignore
 
-# User Guide Source Files
-user_guide_src
-
-# User Guide Compiled Files
+# User Guide source files and compiled files
+user_guide_src export-ignore
 user_guide export-ignore