Adapt ChangeLog: API extended by `net_poll` and `check_pending`
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+API changes
+ * Add function mbedtls_net_poll to public API allowing to wait for a
+ network context to become ready for reading or writing.
+ * Add function mbedtls_ssl_check_pending to public API allowing to check
+ if more data is pending to be processed in the internal message buffers.
+ This function is necessary to determine when it is safe to idle on the
+ underlying transport in case event-driven IO is used.
+
= mbed TLS 2.6.0 branch released 2017-08-10
Security
