Updated changelog

CHANGELOG.md

@@ -25,6 +25,7 @@ changed functionality, fixed bugs).
 - Subprocess commands now write the spawned process PID in a text file, and refuse to run if there
   already is such a file with an alive PID.
 - Log lines produced by subprocesses are now prefixed with 'PID=nnn'.
+- Moved from pip-installing requirements.txt to Pipenv.
 
 
 ## Version 2.1.0 (2018-01-04)