Include package list for Fedora?

I think that Fedora (should you wish to play the "documenting multiple distros" game) has the appropriate GCC available in it's repos as `arm-none-eabi-gcc-cs` and `arm-none-eabi-gcc-cs-c++` meaning that the packages required for can be installed via:

``` console
$ sudo dnf install cmake ninja-build srecord arm-none-eabi-gcc-cs arm-none-eabi-gcc-cs
```

In addition, the `python3-cryptography` and `python3-cffi` packages are needed to allow the `pip3 install yotta` to complete without any native code compilation steps. (Yotta GitHub checkouts appears to fail on Python 3 on my machine but that's a separate issue.)

I can't fully test this locally since a) I'm not NDA-blessed meaning that I can't clone the DAL and b) although being in Cambridge I'm probably minutes away from the nearest micro:bit, I don't have one to hand :). Otherwise, I'd have opened a PR for the README.