Building: Add mention of USE_MBEDTLS

Author: paullouisageneau

BUILDING.md

@@ -19,10 +19,9 @@ $ git submodule update --init --recursive --depth 1
 
 The CMake library targets `libdatachannel` and `libdatachannel-static` respectively correspond to the shared and static libraries. The default target will build tests and examples.
 
-The option `USE_GNUTLS` allows to switch between OpenSSL (default) and GnuTLS, and the option `USE_NICE` allows to switch between libjuice as submodule (default) and libnice.
+Options `USE_GNUTLS` and `USE_MBEDTLS` allow to switch the cryptographic backend to GnuTLS and Mbed TLS respectively, otherwise OpenSSL is selected by default. The option `USE_NICE` allows to switch between libjuice as submodule (default) and libnice as system library.
 
-The option `PREFER_SYSTEM_LIB` allow to link against the system library rather than building all the submodule.
-Options `USE_SYSTEM_SRTP`, `USE_SYSTEM_JUICE`, `USE_SYSTEM_USRSCTP`, `USE_SYSTEM_PLOG` and `USE_SYSTEM_JSON` allow to do the same but per submodule, for libsrtp, libjuice, libusrsctp, Plog and Nlohmann JSON respectively.
+The option `PREFER_SYSTEM_LIB` allows to link against the system library rather than building all the submodule. Options `USE_SYSTEM_SRTP`, `USE_SYSTEM_JUICE`, `USE_SYSTEM_USRSCTP`, `USE_SYSTEM_PLOG` and `USE_SYSTEM_JSON` allow to do the same but per submodule, for libsrtp, libjuice, libusrsctp, Plog and Nlohmann JSON respectively.
 
 If you only need Data Channels, the option `NO_MEDIA` allows to make the library lighter by removing media support. Similarly, `NO_WEBSOCKET` removes WebSocket support.
 
@@ -78,7 +77,7 @@ $ nmake
 
 ## Build directly with Make (Linux only)
 
-The option `USE_GNUTLS` allows to switch between OpenSSL (default) and GnuTLS, and the option `USE_NICE` allows to switch between libjuice as submodule (default) and libnice.
+Options `USE_GNUTLS` and `USE_MBEDTLS` allow to switch the cryptographic backend to GnuTLS and Mbed TLS respectively, otherwise OpenSSL is selected by default. The option `USE_NICE` allows to switch between libjuice as submodule (default) and libnice as system library.
 
 If you only need Data Channels, the option `NO_MEDIA` removes media support. Similarly, `NO_WEBSOCKET` removes WebSocket support.