How to make a buffer have a read-only in Sublime Text 2

The plugin "Toggle the View Read-Only" will do it. It basically does what MattDMo said: when you set the view as read-only, the file can still be changed by another program (or another user), and Sublime Text will pick up those changes. It also has the context menu item you asked for. I like the "Readonly" indicator in status bar.

I didn't test it on Sublime Text 2, but in Sublime Text 3 it works great, and it claims to work on Sublime Text 2 as well.

Yes, this is possible, but you'll have to write a plugin (which actually isn't that hard, especially if you know Python). The API call is view.set_read_only(flag) in the sublime module, where Flag is a boolean. Here's a quick example which checks if a newly-opened file has a certain suffix, and if so sets it to read-only.

import sublime
import sublime_plugin


class MakeViewReadOnlyCommand(sublime_plugin.TextCommand):
    def run(self, edit):
        if self.view.file_name().endswith(".cfg"):
            self.view.set_read_only(True)


class ConfigFileListener(sublime_plugin.EventListener):
    def on_load(self, view):
        view.run_command("make_view_read_only")

Open a new file with Python syntax, copy the code into it, alter it as needed, then save it in your Packages/User directory as make_view_read_only.py. Restart Sublime to load it, and you should be all set. To test if a certain view is read-only, open the console and enter

view.is_read_only()