Readme update - one of the two pending pull requests mentioned has been merged

[hlovdal]

The "pending" pull request Fix for MODBUS_GET_* macro is merged and included upstream.

[i-ky]

Readme actually says:

• This module doesn't work with the upstream libmodbus lib, so patched libmodbus v3.1.4 library is used underneath and shipped as a submodule. Two patches are applied: - Encapsulated Modbus RTU over TCP (libmodbus PR) - Fix for MODBUS_GET_* macro (libmodbus PR)

Fix for MODBUS_GET_* macro is indeed merged and available in libmodbus 3.1.7.

However, the module still ships libmodbus 3.1.4: https://github.com/v-zhuravlev/libmodbus/blob/014c1f35eb1ff42144995f5f150f385064091f41/configure.ac#L14-L16 ...which still needs both patches.

I think to address this issue properly we need to:

• update libmodbus fork from upstream;
• update this repository's .gitmodules;
• remove mentions of merged PR from readme.