Specify that musl-gcc is needed in documentation

[pstavirs]

On Ubuntu 18.04, installing just the musl library - package musl or musl-dev is not enough, even though I had gcc installed and working.

You need to install musl-tools since exodus looks specifically for musl-gcc which is part of musl-tools.

It might be useful to modify the README to specify that musl-gcc is required. The way the current text reads, it appears as if just having the musl library is enough.

Using exodus 2.0.4

Thanks for your wonderful work!