Add BearSSL license info to README.md (esp8266#5211)

Author: earlephilhower

README.md

@@ -128,3 +128,5 @@ ESP8266 core files are licensed under LGPL.
 [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).
+
+[BearSSL](https://bearssl.org) library written by Thomas Pornin, built from https://github.com/earlephilhower/bearssl-esp8266, is used in this project.  It is distributed under the [MIT License](https://bearssl.org/#legal-details).