I have these sentinel stones in my warehouse and I see a description that says they are to donate to my sentinel.
How? Where?? For what???.
If you mean those:
they are used only, if your Sentinel is on 20th lvl (or every 20) and those are working like HB+Gold for standard heroes. It allows Sentinel to get stronger by another 20 lvls.
You can read about it here: Introduction to Guild Turf