Document that we don't require a compiler any more
The upcoming stable release will be provided as wheels, where a compiler
is not necessary to install cocotb.
The current stable version of cocotb requires:
- Python 3.6+
-- Python development packages (Python/C API headers and embedding library)
-- GCC 4.8.1+, Clang 3.3+ or Microsoft Visual C++ 14.21+ and associated development packages
- GNU Make 3+
- An HDL simulator (such as [Icarus Verilog](https://docs.cocotb.org/en/stable/simulator_support.html#icarus-verilog),
[Verilator](https://docs.cocotb.org/en/stable/simulator_support.html#verilator),
