Explicitly state the dependencies on make and gcc

[LuisaAPF]

Hi, when trying to compile the dependencies on a scenic project, I got the following error: could not compile dependency :scenic, "mix compile" failed. You can recompile this dependency with "mix deps.compile scenic", update it with "mix deps.update scenic" or clean it with "mix deps.clean scenic" ** (Mix) "make" not found in the path. If you have set the MAKE environment variable, please make sure it is correct.

I found out later that I needed to have make and gcc installed, which I didn't have on my Ubuntu 18.04. Although the error message gives a hint to the solution, it is not very explicit. So I would like to suggest explicitly adding these dependencies to the README.

[Eiji7]

@LuisaAPF You do not have all OS dependencies. Please read to README.md of scenic_driver_glfw (as it's a default driver).

In Ubuntu it's really helpful (for lots of projects) to install: build-essential. In source-based distributions there is no even need to mention that as things like gcc or make are always installed as dependencies of lots of packages.

[LuisaAPF]

Thanks for your reply, @Eiji7 . I know these packages already come by default in many distributions, but I thought it would be worth it to register here, in case someone else runs into the same issue. If you judge it's not relevant to put this info on the repository's README, feel free to close the issue ✌️