How to manage a copyright notice in an open source project?

Question

I'm working on, and adding to, a GitHub-hosted project that includes this LICENCE.md (apparently the MIT licence verbatim):

Copyright (c) 2012 [Acme Corp]

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

I've made changes, improvements etc (say 10% of the code, in ballpark figures) and publish the code to my own GitHub fork.

What should I do with this copyright notice? I'd like to update it (eg, just to add the name of my own organisation), but it says not to. How are these things normally managed? Add a separate copyright file?