Adjust README disclaimer wording

[paulhowardarm]

Summary

As part of production readiness, the wording of the headline disclaimer in the README can be adjusted.

Details

New wording TBD, but we should indicate that the software is suitable for production systems provided that secure deployment guidelines are followed. We should probably reference these guidelines along with the threat model. We probably need to indicate that different parts of the system will have been tested to different levels, because wholesale system testing is generally the responsibility of the consuming software product or service. We should by this point be confident that there will not be further breaking changes in wire protocol.

I would suggest the heading "Disclaimer" can be changed to "Using Parsec in Production"

[paulhowardarm]

The aim of the Parsec Production Ready milestone is to get the project into a state where these disclaimers can be removed or reworded, so it follows that this should be the final item that we do once all other PPR issues are done.