Deprecate mention + Add link to https://github.com/jonathansharp/CO2-System-Extd?

[briochemc]

It would be good in my opinion if a link to CO2-System-Extd was provided here (somewhere in the ReadMe), especially if CO2SYS-MATLAB is not maintained anymore, in which case a note at the top of the ReadMe would also be welcome.

(I missed that there was another more up-to-date version out there myself.)