Add informations about generatin language files

[Orholt]

Hi, the documentation needs more information about using res/lang.py, what it does etc.

Could you guys assign to me ? (I do not see option for self assign)

[rustdesk]

We add new items in template.rs, res/lang.py will add these items to the other language files automatically.

[rustdesk]

We have more than 20 languages, manually adding one by one is not easy, so we write res/lang.py to help us.

[Orholt]

Ok, I'll make a section in documentation about this, thanks for comments.