Warn more explicitly about RFC5054 compatibility

[awakecoding]

I have integrated this SRP6a implementation in my code base in 2017, only to realize now that it never was interoperable with the RFC5054 standard. I don't recall if the note was explicit enough back then, but even today, most people will default to using the master branch. Instead of a soft "NOTE" in the readme, this should be some sort of scary warning that can't be missed.

I understand the point of maintaining backwards compatibility for older implementations that used this code before RFC5054, but shouldn't the master branch conform to RFC5054, with a pre-RFC5054 compatibility branch instead of the other way around? It's not like people fetch the code directly from the master branch every single time they build their project.