docs: update readme light of sablier v2

[PaulRBerg]

Two changes:

• Remove the note about the precision error in Sablier; this is no longer the case in V2, see our updated function _calculateStreamedAmount
• I updated the gas cost table with a more realistic figure. It is misleading to say that creating a LlamaPay stream costs only 70k gas, because that's only the cost of creating an empty, unfunded stream, which is useless. A useful LlamaPay stream has to be funded, and that costs ~158,560 based on my calculations: 72,204 and 86,356

[PaulRBerg]

Hey @0xngmi, @skellet0r, can you have a look at this PR, please?