Add a library.json file for PlatformIO to use

[egnor]

PlatformIO is an increasingly popular open source build system for embedded development, supporting many frameworks, targets and environments. (I am not affiliated in any way with PlatformIO, I'm just a user.)

PlatformIO has a library registry that describes a large number of libraries which can potentially be reused. (This is analogous to Arduino's library registry, but more flexible and general.)

PlatformIO can pull directly from the git repo, but it needs a library.json file giving a brief synopsis of the library and some basic build instructions. As you can see it's quite straightforward, especially since this particular library is very easy to build.

This doesn't touch ANY other code within the repository, so I don't think unit testing is needed. (We could potentially add a test to verify that library.json exists and is valid JSON?) I have manually tested by pulling it into a PlatformIO build from my fork.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Thank you for your consideration!

[aggarg]

coreMQTT can be used with a variety of build systems. We only include CMake files in the repository to avoid proliferation of build system files. I'd suggest to host this JSON file in a separate repo which can submodule coreMQTT. If you decide to host this, please share with us and we will include this in our readme so that the interested users can find it.

[aggarg]

I am closing this PR. Feel free to reopen if you need anything from us.