Add Powers of two table

README.md

@@ -1644,3 +1644,26 @@ Security is a broad topic.  Unless you have considerable experience, a security
 
 * [Security guide for developers](https://github.com/FallibleInc/security-guide-for-developers)
 * [OWASP top ten](https://www.owasp.org/index.php/OWASP_Top_Ten_Cheat_Sheet)
+
+## Appendix
+
+You'll sometimes be asked to do 'back-of-the-envelope' estimates.  For example, you might need to determine how long it will take to generate 100 image thumbnails from disk or how much memory a data structure will take.  The **Powers of two table** and **Latency numbers every programmer should know** are handy references.
+
+### Powers of two table
+
+```
+Power           Exact Value         Approx Value        Bytes
+---------------------------------------------------------------
+7                             128
+8                             256
+10                           1024   1 thousand           1 KB
+16                         65,536                       64 KB
+20                      1,048,576   1 million            1 MB
+30                  1,073,741,824   1 billion            1 GB
+32                  4,294,967,296                        4 GB
+40              1,099,511,627,776   1 trillion           1 TB
+```
+
+#### Source(s) and further reading
+
+* [Powers of two](https://en.wikipedia.org/wiki/Power_of_two)