Add ChangeLog entry for mbedtls_ssl_write() docs
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.x.x released xxxx-xx-xx
+
+Bugfix
+ * Clarify documentation for mbedtls_ssl_write() to include 0 as a valid
+ return value. Found by @davidwu2000. #839
+
= mbed TLS 2.11.0 branch released 2018-06-18
Features
