Document constants used in AvalancheTest.cpp and if they should be adjusted based on reps or state size

[x448]

First of all, thanks for everything you've already done in the README.

It may be useful to document how some constants should be adjusted based on state size and/or reps. Thanks again what you've already done.

https://github.com/demerphq/smhasher/blob/076288788b1c692f30476e91798f312fae7df8e0/AvalancheTest.cpp#L275-L276

https://github.com/demerphq/smhasher/blob/631955c319716503929b47d392de104d1abfaf07/RunTests.cpp#L145-L153