Reordered install instructions: moved `tpm` higher.
Reordered install instructions: moved `tpm` higher.

Since it's the most tmux-native install option, I've moved it higher in
the list of installation options.

Also adds @nhdaly to the Contributions section per @thewtex's suggestion
in
[#43](https://github.com/thewtex/tmux-mem-cpu-load/issues/43#issuecomment-244767735).

thewtex/tmux-mem-cpu-load,thewtex/tmux-mem-cpu-load,thewtex/tmux-mem-cpu-load,thewtex/tmux-mem-cpu-load