feat: added overview of staking

docs/usage/stake.md

@@ -1,4 +1,30 @@
-# Stake and Unstake
+# Staking
+
+In Walrus, anyone can delegate stake to storage nodes and, by doing so, influence, which storage
+nodes get selected for the committee in future epochs, and how many shards these nodes will hold.
+Shards are assigned to storage nodes every epoch, roughly proportional to the amount of stake
+that was delegated to them. By staking with a storage node, users also earn rewards, as they
+will receive a share of the storage fees.
+
+Since moving shards from one storage node to another requires transferring a lot of data and
+storage nodes potentially need to expand their storage capacity, the selection of the committee
+for the next epoch is done ahead of time, in the middle of the previous epoch. This provides
+sufficient time to storage node operators to provision additional resources, if needed.
+
+For stake to affect the shard distribution in epoch `e` and become "active", it must be staked
+before the committee for this epoch has been selected, meaning that it has to be staked before
+the midpoint of epoch `e - 1`. If it is staked after that point in time, it will only influence
+the committee selection for epoch `e + 1` and thus only become active, and accrue rewards, in
+that epoch.
+
+Unstaking has a similar delay: because unstaking funds only has an effect on the committee in
+the next committee selection, the stake will remain active until that committee takes over.
+This means that, to unstake at the start of epoch `e`, the user needs to "request withdrawal"
+before the midpoint of epoch `e - 1`. Otherwise, i.e., if the user unstakes after this point,
+the stake will remain active, and continue to accrue rewards, throughout epoch `e`, and the
+balance and rewards will be available to withdraw at the start of epoch `e + 1`.
+
+## How to stake
 
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