add "README" file containing notes for running "fsk_modem_app.xml"

What this MR does / why we need it:

This MR adds a "README" file in the "applications/fsk_dig_radio_ctrlr" directory. The file contains notes on running the "fsk_modem_app.xml" application.

Steps to complete before submitting MR:

• I have read Contribution guidelines

• I have ensured I have a changelog written in the imperative form that is meaningful and accurately summarizes the work done

• I have added the release notes label if applicable (more information below)

• I have ensured the workflow labels are up to date and will continue to do so up until this work is merged into develop

• I have added a category, type, and target release label along with all other labels as need be

• My branch is up to date with develop. If it is not, then I have ensured my work does not conflict with other work

• I agree my bugfix MR does not include new features/enhancements

• I represent that bugfixes have been locally tested against the most recent major.minor release in which the bug exists

• (REVIEWER ONLY) I have thoroughly gone through the above steps to ensure that they have been followed

Release Notes

N/A

Changelog

• task(doc,osp): add README file containing notes for running fsk_modem_app.xml

Which issue(s) this MR closes

Closes #73 (closed)