Add additional dependencies to README.md

[cscott]

Add rustup, cargo-make and elf2uf2-rs as dependencies to ensure that a user with no prior rust installation can build the firmware.

Closes #103

[Bieitodlf]

I realize this is from a while ago. I was going to add the cargo-make dependency to the application dependency list and I saw this PR, which adds it to the firmware dependency list. I can create a separate PR, but it may make more sense to update this one?

[cscott]

It also turns out that you need to do the equivalent of apt-get install pkg-config libudev-dev if you're installing from a clean system, otherwise cargo install elf2uf2-rs will fail.

[JohnAZoidberg]

I realize this is from a while ago. I was going to add the cargo-make dependency to the application dependency list and I saw this PR, which adds it to the firmware dependency list. I can create a separate PR, but it may make more sense to update this one?

I added it as well in the tool section, thanks!