message: Add note about file/socket limit for MacOS tests of libindy.
diff: In order to run local nodes on MacOS, it may be necessary to set up port mapping between the Docker container
  and local host. Follow the instructions in [Indy SDK README](https://github.com/hyperledger/indy-sdk#how-to-start-local-nodes-pool-with-docker)
  +
  +# IOError while running of whole set of tests on MacOS
  +
  +There is a possible case when some tests are failed if whole set of tests is run (`cargo test`).
  +But failed tests will be successful in case of separate runs.
  +If an error message like `IOError` `Too many open files` is present in logs when fails can be fixed by changing default limit.
  +
  +`ulimit -n <new limit value>`
  +
  +https://jira.hyperledger.org/browse/IS-1038
  