README: Update

README.md

@@ -33,3 +33,31 @@ julia generate_functions.jl
 * [wiki](https://docs.einsteintoolkit.org/et-docs/Prolongation)
 
 * [pseudocode](https://docs.google.com/document/d/1M65w_8keIf6Ypas43YXf1CA-tbuhOssFPpBPlGS1tD0/edit)
+
+## RK4 Steps
+
+* Initialise
+
+* March the first substep for all levels (coarse to fine)
+
+```bash
+ ------
+
+        ------
+               ------
+ ------ ------ ------
+```
+
+* March the other substeps to the same time slice
+
+```julia
+for s = 2:2^(lmax-1)  # from second to final substeps of the finest level
+    for l = 2:lmax  # march all levels execpt the coarest (coarset to fine)
+        if l == lmax || (levs[l].time == levs[l+1].time)
+            if l < lmax
+                Restriction  # from l+1 to l
+            Prolongation  # from l-1 to l
+            rk4(levs[l])
+```
+
+* Restriction all levels