Add resources documentation

Author: aledeg

doc/resources.md

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+# Resources
+
+## Flickr API
+1. [API keys](https://www.flickr.com/services/api/keys/)
+1. [REST request format](https://www.flickr.com/services/api/request.rest.html)
+1. [Photo image URLs](https://www.flickr.com/services/api/misc.urls.html)
+1. [flickr.photos.getSizes](https://www.flickr.com/services/api/flickr.photos.getSizes.html)
+
+## Gfycat API
+1. [API Reference](https://developers.gfycat.com/api/#getting-info-for-a-single-gfycat)
+
+## Imgur API
+1. [Authorization and OAuth](https://apidocs.imgur.com/#authorization-and-oauth)
+1. [Album images](https://apidocs.imgur.com/#7dde894b-a967-4419-9be2-082fbf379109)
+
+## RedGIFs API
+1. [API Reference](https://api.redgifs.com/docs/index.html)
+1. [Request a RedGIFs API key](https://docs.google.com/forms/d/e/1FAIpQLSf-jnx_BA_y_I3cASpSDrh8dxS18a0r56Bp1T5fTNj7JI4b2g/viewform?hl=en)
+1. [Requesting media links](https://github.com/Redgifs/api/wiki/Requesting-media-links)