docs: update readme with simplified installation instructions
docs: update readme with simplified installation instructions

I tested installation in a virtualenv by doing the following:

    pip install --pre https://github.com/posborne/pyOCD/archive/simplify-installation.zip

The `--pre` option is required on Linxu as all of the artifacts for
pyusb on pypi have a beta or alpha prefix.

c1728p9/pyOCD,devanlai/pyOCD,tgarc/pyOCD,bridadan/pyOCD,bridadan/pyOCD,matthewelse/pyOCD,mesheven/pyOCD,oliviermartin/pyOCD,adamgreen/pyOCD,devanlai/pyOCD,mesheven/pyOCD,geky/pyOCD,tgarc/pyOCD,adamgreen/pyOCD,c1728p9/pyOCD,wjzhang/pyOCD,matthewelse/pyOCD,flit/pyOCD,pyocd/pyOCD,tgarc/pyOCD,wjzhang/pyOCD,adamgreen/pyOCD,c1728p9/pyOCD,mbedmicro/pyOCD,0xc0170/pyOCD,molejar/pyOCD,molejar/pyOCD,flit/pyOCD,pyocd/pyOCD,geky/pyOCDgdb,wjzhang/pyOCD,molejar/pyOCD,mesheven/pyOCD,0xc0170/pyOCD,oliviermartin/pyOCD,devanlai/pyOCD,geky/pyOCD,0xc0170/pyOCD,bridadan/pyOCD,geky/pyDAPLink,matthewelse/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,oliviermartin/pyOCD