README: Add todo.

README.md

@@ -6,7 +6,9 @@ I'm focusing only on the "flattening" step which I like to call "apply_gravity"
 
 I'm focusing only in flattening a single huge pile of sand placed in the middle.
 
-Here the performance I'm getting on an intel `i9-9980HK`:
+Here the performance I'm getting on an intel `i9-9980HK` to flatten a
+100_000 sand grain tower:
+
 
 ## sandpile_quarter.c
 
@@ -16,6 +18,7 @@ for simplicity, so it could be enchanced again.
 
 > Mean +- std dev: 208 ms +- 5 ms
 
+
 ## sandpile_1d.c
 
 This one does not rely on pointers of pointers to represent a 2d
@@ -78,3 +81,9 @@ a FIFO of points to fire is kept up to date each time a cell is
 firered.
 
 > Mean +- std dev: 2.81 sec +- 0.05 sec
+
+
+## TODO
+
+- Try a CUDA implementation.
+- Try a multithreaded implementation (beware of the lock bottleneck!).