Add rough memory usage

README.md

@@ -5,6 +5,7 @@ feature | weaselab | fdb |
 |cheaper iteration*|❌|✅|
 |efficient bulk queries|✅|❌|
 |entries per set (worst-case)|1|3|
+|memory-usage (rough)|%40|%100|
 |node gc|mark-and-sweep|reference-counting|
 |pointer size|32-bit|64-bit|
 |purge strategy|scan|search|