readme: mention axTLS library

fixes #3038

Author: igrr

README.md

@@ -127,3 +127,5 @@ ESP8266 core files are licensed under LGPL.
 [SPI Flash File System (SPIFFS)](https://github.com/pellepl/spiffs) written by Peter Andersson is used in this project. It is distributed under MIT license.
 
 [umm_malloc](https://github.com/rhempel/umm_malloc) memory management library written by Ralph Hempel is used in this project. It is distributed under MIT license.
+
+[axTLS](http://axtls.sourceforge.net/) library written by Cameron Rich, built from https://github.com/igrr/axtls-8266, is used in this project. It is distributed under [BSD license](https://github.com/igrr/axtls-8266/blob/master/LICENSE).