Create MessageBuilder.md

Move MessageBuilder class reference to its own page.

Author: Amanda Butler

docs/api/nfc/MessageBuilder.md

@@ -0,0 +1,35 @@
+## MessageBuilder
+
+Mbed OS provides this API to construct NDEF messages, the common data format exchange for NFC messages. The class `mbed::nfc::ndef::MessageBuilder` builds an NDEF message into a user-provided buffer. `URI`, `Text` and `Mime` types can be serialized in the builder with the help of the member function `append_as_record`.
+
+### MessageBuilder class reference
+
+[![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1nfc_1_1ndef_1_1_message_builder.html)
+
+### MessageBuilder example
+
+```
+#include "nfc/ndef/MessageBuilder.h"
+
+using mbed::nfc::ndef::MessageBuilder;
+using mbed::nfc::ndef::common::Text;
+using mbed::nfc::ndef::common::URI;
+
+size_t build_ndef_message(const Span<uint8_t> &buffer) {
+    MessageBuilder builder(buffer);
+
+    URI uri(URI::HTTPS_WWW, span_from_cstr("mbed.com"));
+    Text text(Text::UTF8, span_from_cstr("en-US"), span_from_cstr("Mbed website"));
+
+    uri.append_as_record(builder);
+    text.append_as_record(builder, /* last record */ true);
+
+    return builder.get_message().size();
+}
+```
+
+### Related content
+
+- [MessageParser](messageparser.html) API reference.
+- [SimpleMessageParser](simplemessageparser.html) API reference.
+- [NFC architecture](/docs/development/reference/nfc-technology.html).