added docs for converter

[selwyn96]

This pull request adds the documentation outlining the limitation of the current Qkeras converter.

[maltanar]

Hi @selwyn96 , thanks for the PR. To keep this consistent with the rest of the qonnx docs (viewable online on https://qonnx.readthedocs.io/ or can be built with tox -e docs) I'd like to request a couple of changes:

• Instead of having this as a standalone Markdown file, could you add this as ReStructured Text (rst) to the documentation under https://github.com/fastmachinelearning/qonnx/tree/main/docs and link it from the toctree under https://github.com/fastmachinelearning/qonnx/blob/main/docs/index.rst ?
• Can you elaborate a bit more on the stability issue for ternary quantization? What can the user expect, what is the symptom/bug that will arise here?
• Can you add a link to the relevant test(s) in the test suite that the users can look at as reference?