Add brief cheatsheet to docs

I'm realizing that in addition to the right documentation already accompanying Tiled, we should distill the essentials into a short cheatsheet.  Something like these:
* [Conda](https://docs.conda.io/projects/conda/en/4.6.0/_downloads/52a95608c49671267e40c689e0bc00ca/conda-cheatsheet.pdf)
* [PyCharm](https://resources.jetbrains.com/storage/products/pycharm/docs/PyCharm_ReferenceCard.pdf)