README.md: note that master branch is work in progress to migrate to python3

- to run with Python2.x, use branch v1.2-python2

README.md

@@ -12,6 +12,18 @@ It requires that `jsonrpcs` module of Kamailio is loaded and configured to liste
 on a Unix domain socket or FIFO file. The way to interact with Kamailio has to be set
 inside `kamcli` config file (kamcli.ini).
 
+### Important Note ###
+
+The master branch of `kamcli` is under ongoing work to upgrade the code to work with Python3.
+Migration to Python3 is required because Python2 has been deprecated and it is going to be
+removed from major Linux distributions.
+
+Therefore, for the moment, not all commands may work with Python3, if you find any problem,
+file a report on Issue Tracker.
+
+To run `kamcli` with Python2.x, use the git branch `v1.2-python2`. The branch will be kept
+for a while, but likely there will be no new features added to it.
+
 ### Features
 
 The prototype of using **kamcli** is: