Update README.md to include cmp submodule info

Author: Tha14

README.md

@@ -39,6 +39,8 @@ on the website, where they are updated at least once every 24 hours:
 
 Detailed installation instructions can be found in [INSTALL.md](INSTALL.md).
 
+Be advised that due to the addition of `cmp` as a submodule, you now also need to initialize the git submodules required by toxcore. This can be done by cloning the repo with the following command: `git clone --recurse-submodules https://github.com/Toktok/c-toxcore` or by running `git submodule update --init` in the root directory of the repo. 
+
 In a nutshell, if you have [libsodium](https://github.com/jedisct1/libsodium)
 installed, run: