Modifies Lightweight example to with PSL changes

[krd-rti]

Summary

Modifies Lightweight security example to work with PSL

Details and comments

Added content about PSL in README.md and new PSL-related properties in the QoS profile. Note, the changes are optional, since the values of these properties are the default values. Example works without them if the intent is to work with openssl lib.

####Observations

• Changes for wolfssl are not necessary, since the original example does not support it.
• The example is built statically, yet the QoS changes are compiled in. No need to modify the code.

Checks

• [ X] I have updated the documentation accordingly.
• [ X] I have read the CONTRIBUTING document.