Add code coverage badge to README.md

[daka1510]

Summary

With #144 we started reporting code coverage metrics to coveralls.io. This PR adds a badge to the README so that interested parties can easily navigate to and review coverage metrics

Details and comments

Fixes #22

[rathishcholarajan]

LGTM!

[coveralls]

Pull Request Test Coverage Report for Build 1847910704

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 189 unchanged lines in 17 files lost coverage.
• Overall coverage decreased (-5.4%) to 58.03%

---

Files with Coverage Reduction	New Missed Lines	%
qiskit_ibm_runtime/accounts/account.py	1	93.02%
qiskit_ibm_runtime/ibm_backend.py	1	60.0%
qiskit_ibm_runtime/runtime_program.py	1	88.68%
qiskit_ibm_runtime/api/auth.py	2	74.19%
qiskit_ibm_runtime/utils/backend_decoder.py	2	47.54%
qiskit_ibm_runtime/api/clients/base.py	4	88.03%
qiskit_ibm_runtime/utils/converters.py	6	18.97%
qiskit_ibm_runtime/api/rest/program_job.py	9	60.87%
qiskit_ibm_runtime/api/rest/cloud_backend.py	11	37.5%
qiskit_ibm_runtime/api/session.py	11	70.54%

Totals
Change from base Build 1846800755:	-5.4%
Covered Lines:	2009
Relevant Lines:	3462

---

💛 - Coveralls