Add information in readme and documentation on installations of pyquil, qiskit for non devs

[nathanshammah]

Right now, the readme and documentation simply refer to a development install for users interested in pyquil and qiskit.

However, imagine that a non-dev user is interested (as a user, not as a contributor to mitiq) in using mitiq + pyquil or mitiq + qiskit, then he/she has no clear information on the version requirements for these libraries, i.e. from the requirements.txt

# third-party integration
pyquil~=2.18.0
qiskit~=0.16.2

Such user would not be able to run the tests without these information.

Proposed feature

Add information about this in the readme and in the documentation, e.g.,

"""

Using mitiq with Qiskit

If you are willing to use mitiq to mitigate Qiskit circuits, make sure to have qiskit~=0.16.2 installed:

pip install qiskit~=0.16.2 mitiq

"""
Also, if we decide to do this, we could reference this back in the documentation's guide when such 3rd party executors are mentioned.

[willzeng]

+1