Invoice limit into the Documentation

[Ly0n]

Minimum 50 trees invoice limit

[Ly0n]

Jonathan:

Chase Africa: 20$
TIST: 20$
Sustainable Harvest International: 50$
Conserve Natural Forests : 20$

@tjarkdoering We need to update the disclaimer.

[tjarkdoering]

See my latest PR. I have a proposition in there.