Write readme for version update

README.md

+# Updating PHP Version
+
+```
+# Example: from 7.1 to 7.3
+
+ascr role php
+
+# - Move FPM Conf from 7.1 to 7.3
+# - Remove www FPM Conf from 7.3
+# - Stop FPM 7.1
+# - Restart FPM 7.3
+
+a -m service -a "name=php7.1-fpm state=stopped enabled=no"
+a -m service -a "name=php7.3-fpm state=started enabled=yes"
+ascr jailkit-upgrade
+
+etckeeper commit -m "After PHP update to PHP 7.3"
+```