A more complicated way to do it is to add a folder that contains the original LICENSE file or files. Sometimes there is more than one license, or the license texts differ. In that case, you must preserve all the different variants, even if they all call themselves MIT.
Then, you can optionally add your additional own LICENSE file * only iff* it is compatible with all existing LICENSES. In the case of the MIT license, you may relicense, sublicense, or use a different license in addition, provided it is MIT-compatible. With e.g. GPL you can't. Note that you still have to preserve all the original LICENSE files in the repo.
