README: add a simple readme to explain how to update mavlink message set

HOWTO-change-mavlink-msg.md

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+### Updating MAVLink Message Set
+
+To update the MAVLink message set, you will need to rebuild both mavlink2rest and [rust-mavlink](https://github.com/mavlink/rust-mavlink). As of August 2023, this process is not straightforward and requires following specific instructions.
+
+Firstly, you need to set up a working RUST environment. Follow the official instructions at https://www.rust-lang.org/ to set up your Rust environment.
+
+Next, obtain the source code of rust-mavlink and update its MAVLink submodule to acquire the correct MAVLink revision:
+```sh
+git clone [email protected]:mavlink/rust-mavlink.git --recursive
+```
+
+Retrieve the source code of mavlink2rest as well:
+```sh
+git clone https://github.com/khancyr/mavlink2rest --recursive
+```
+
+Select the version of mavlink2rest you desire. Open the `Cargo.toml` file to determine the corresponding version of rust-mavlink. Then, checkout the desired version of rust-mavlink, for example:
+```sh
+cd rust-mavlink/
+git checkout 0.10.2
+```
+
+Now, you can edit the MAVLink message definitions located in `mavlink/message_definitions/v1.0`.
+
+Compile rust-mavlink with the desired message set you wish to support. In this example, we'll use the default `ardupilotmega.xml`:
+```sh
+cargo install --path . --features="ardupilotmega emit-extensions"
+```
+
+Once the compilation is finished, return to the `Cargo.toml` file of mavlink2rest and update the path to the local rust-mavlink library. 
+
+Before:
+```toml
+mavlink = { git = "https://github.com/mavlink/rust-mavlink", rev = "0.10.2", features = [ "ardupilotmega", "emit-extensions"] }
+```
+
+After:
+```toml
+mavlink = { path="/home/khancyr/Workspace/rust-mavlink", features = [ "ardupilotmega", "emit-extensions"] }
+```
+
+This change points to the updated rust-mavlink library containing the updated MAVLink messages.
+
+Finally, compile mavlink2rest:
+```sh
+cargo install --path .
+```
+
+Your newly compiled mavlink2rest will now comprehend your newly added messages.
+