Mention SglPPow in info messages, README and documentation

[olexandr-konovalov]

Since SglPPow is not specified as suggested package (see #11 and #14), it's important that it is mentioned in the README and in the manual.

[olexandr-konovalov]

Furthermore, one could modify functions to produce info messages when requested order is p^7 for p>11 or 3^8 and recommend to load SglPPow. I think this will completely cover the needs of informing the user about SglPPow without expanding dependencies. I will try to submit a pull request.

[fingolfin]

I still am not sure I agree with this idea. What makes sglppow special compared to all those other packages which extend and improve GAP, yet we don't bother to add info messages and notices everywhere to suggest to people that they may want to load them?

[olexandr-konovalov]

Answering the part on what makes it special: it's very tight integration with the small group library by making groups accessible via SmallGroup. Also, it's a relatively new package. Doing what I've suggested will improve user-friendliness.