Update README.md

README.md

@@ -80,6 +80,10 @@ Ans. No, storing a small number in a uint8 variable is not cheaper than storing
 
 - Use mapping whenever possible, it's cheap instead of the array
 - But an array could be a good choice if you have a small array
+- Fixed size variables are always cheaper than dynamic ones.
+- If we know how long an array should be, we specify a fixed size
+- This same rule applies to strings. A string or bytes variable is dynamically sized; we should use a byte32 if our string is short enough to fit.
+- If we absolutely need a dynamic array, it is best to structure our functions to be additive instead of subractive. Extending an array costs constant gas whereas truncating an array costs linear gas.
 
 ## Functions