Create README.md

Author: igrr

README.md

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+Replacement for Espressif's libssl, kept as close as possible to [axTLS](http://axtls.sourceforge.net/) source.
+Currently based on axTLS 1.4.9, will be upgraded to 1.5.3.
+
+This is not a self-sufficient library. Application has to provide the following symbols (list not complete yet):
+```
+malloc
+calloc
+free
+abort
+gettimeofday
+time
+ctime
+printf
+vprintf
+```
+
+Additionally, functions for non-blocking TCP socket reads and writes have to be provided (details TBD).
+
+To build, add xtensa toolchain to your path, and run `make`.
+
+See [LICENSE](LICENSE) file for axTLS license.