add FASTDDS_BUILTIN_TRANSPORTS env value doc. (#750)

Signed-off-by: Tomoya.Fujita <[email protected]>

README.md

@@ -42,6 +42,7 @@ However, `rmw_fastrtps` offers the possibility to further configure Fast DDS:
 * [Full QoS configuration](#full-qos-configuration)
 * [Change participant discovery options](#change-participant-discovery-options)
 * [Enable Zero Copy Data Sharing](#enable-zero-copy-data-sharing)
+* [Large data transfer over lossy network](#large-data-transfer-over-lossy-network)
 
 ### Change publication mode
 
@@ -306,6 +307,26 @@ In order to achieve a Zero Copy message delivery, applications need to both enab
     </profiles>
     ```
 
+### Large data transfer over lossy network
+
+Out of the box, Fast DDS uses UDPv4 for the data communication over the network.
+Although UDP has its own merit for realtime communications, with many applications relying on UDP, depending on application requirements, a more reliable network transmission may be needed.
+Such cases included but are not limited to sending large data samples over lossy networks, where TCP's builtin reliability and flow control tend to perform better.
+
+Because of this reason, Fast DDS provides the possibility to modify its builtin transports via an environmental variable `FASTDDS_BUILTIN_TRANSPORTS`, allowing for easily changing the transport layer to TCP when needed:
+
+```bash
+export FASTDDS_BUILTIN_TRANSPORTS=LARGE_DATA
+```
+
+This `LARGE_DATA` mode adds a TCP transport for data communication, restricting the use of the UDP transport to the first part of the discovery process, thus achieving a reliable transmission with automatic discovery capabilities.
+This will improve the transmission of large data samples over lossy networks.
+
+> [!NOTE]
+> The environmental variable needs to be set on both publisher and subscription side.
+
+For more information, please refer to [FASTDDS_BUILTIN_TRANSPORTS](https://fast-dds.docs.eprosima.com/en/latest/fastdds/env_vars/env_vars.html#fastdds-builtin-transports).
+
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