Add flat state specs to resharding NEP (#570)

Filling the specification section about flat state.

neps/nep-0568.md

@@ -73,6 +73,37 @@ post-processing, as long as the chain's view reflects a fully resharded state.
 
 ### State Storage - Flat State
 
+Flat State is a collection of key-value pairs stored on disk and each entry
+contains a reference to its ShardId. When splitting a shard, every item inside
+its Flat State must be correctly reassigned to either one of the new children;
+due to technical limitations such operation can't be completed instantaneously.
+
+Flat State main purposes are allowing the creation of State Sync snapshots and
+the construction of Mem Tries. Fortunately, these two operations can be delayed
+until resharding is completed. Note also that with Mem Tries enabled the chain
+can move forward even if the current status of Flat State is not in sync with
+the latest block. 
+
+For the reason stated above, the chosen strategy is to reshard Flat State in a
+long-running background task. The new shards' states must converge with their
+Mem Tries representation in a reasonable amount of time.
+
+Splitting a shard's Flat State is performed in multiple steps:
+1) A post-processing 'split' task is created during the last block of the old
+   shard layout, instantaneously. 
+2) The 'split' task runs in parallel with the chain for a certain amount of
+   time. Inside this routine every key-value pair belonging to the shard being
+   split (also called parent shard) is copied into either the left or the right
+   child Flat State. Entries linked to receipts are handled in a special way.
+3) Once the task is completed, the parent shard Flat State is cleaned up. The
+   children shards Flat States have their state in sync with last block of the
+   old shard layout.
+4) Children shards must apply the delta changes from the first block of the new
+   shard layout until the final block of the canonical chain. This operation is
+   done in another background task to avoid slowdowns while processing blocks.
+5) Children shards Flat States are now ready and can be used to take State Sync
+   snapshots and to reload Mem Tries.
+
 ### State Storage - State
 
 ### Stateless Validation