PHP SDK library to include in Wiki

[ChrisHardie]

I released a minimal implementation of the CrowdTangle API in PHP:

https://github.com/ChrisHardie/crowdtangle-api

I thought it could be helpful to include a link to it in the Wiki documentation here, maybe a in a new "Libraries and SDKs" section? I can't submit a PR for a Wiki page but let me know if it would be helpful to draft that section.

(I have completed the Meta CLA, FWIW.)

[TianminH]

Hey Chris,

Thanks for creating such a nice PHP library. Could you please post a draft, in thee same time, I will test the library and reach out to the team who's responsible for maintaining the wiki.

Cheers,
Tianmin

[ChrisHardie]

Sure, thank you. Here's a short draft:

### Libraries and SDKs

Below are some available libraries and SDKs to facilitate integrating the API into your application. These tools are provided by third parties and are not endorsed or managed by CrowdTangle:

* [PHP composer package](https://github.com/ChrisHardie/crowdtangle-api)