update readme with advantages of tiempo

README.md

@@ -154,6 +154,25 @@ day in the computer's timezone. Add `Z` or an offset to specify the timezone.
 * `1h30m ago`
 * `two hours thirty minutes ago`
 
+## Why did you write this instead of improving timetrap?
+
+* timetrap is hard to install, hard to keep updated (because of ruby). Once this
+  tools is finished you can get (or build) a binary, put it somewhere, and it
+  will just work forever in that machine.
+* timetrap is slow (no way around it, because of ruby), some commands take up to
+  a second. Tiempo always feels snappy.
+* needed major refactor to fix the timezone problem (in a language I'm not
+  proficient with). I was aware of this problem and designed tiempo to store
+  timestamps in UTC while at the same time being able to work with a database
+  made by timetrap without messing up.
+
+### Other advantages
+
+* Columns are always aligned
+* Fixed some input inconsistencies
+* cli interface is much better (ask -h for any command)
+* man page (to do)
+
 ## How to build
 
 You need [rust](https://rustup.rs), then clone the repo and simply run