add MessageBuilder example

Author: mprse

APIs_NFC/MessageBuilder/README.md

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+## MessageBuilder Example
+
+This MessageBuilder example shows how to build 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`.

APIs_NFC/MessageBuilder/main.cpp

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+
+#include "mbed.h"
+#include "nfc/ndef/MessageBuilder.h"
+#include "nfc/ndef/common/URI.h"
+#include "nfc/ndef/common/Text.h"
+#include "nfc/ndef/common/util.h"
+
+using mbed::nfc::ndef::MessageBuilder;
+using mbed::nfc::ndef::common::Text;
+using mbed::nfc::ndef::common::URI;
+using mbed::nfc::ndef::common::span_from_cstr;
+
+int main()
+{
+    uint8_t message_data[128];
+    const Span<uint8_t, 128> buffer(message_data, sizeof(message_data));
+
+    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);
+
+    printf("Message size: %d bytes\r\n",  builder.get_message().size());
+}