Issue title: Specify jQuery version in readme
 Issue body: **Summary**

I spent a fair amount of time debugging why the board wasn't working as expected. It turns out jQuery v3.3.1 was not up to date enough. I see that the demo websites are running jQuery v3.4.1

**Proposal**

Can you specify the minimum jQuery version in the readme?
 Comments: 
 Comment 0: Seconded. It took me a full hour to get it working even though I've read the instructions on the website. Why is this still not inside the README? Is this project not being maintained anymore?
 Comment 1: > Can you specify the minimum jQuery version in the readme?

Done with [commit f093aaf4](https://github.com/oakmac/chessboardjs/commit/f093aaf4e99e5fcfc3a8f156ea5338205869cc28)

I'm sorry y'all had trouble with this; I agree it should be clear in the documentation. The minimum jQuery version is specified in the [package.json](https://github.com/oakmac/chessboardjs/blob/f093aaf4e99e5fcfc3a8f156ea5338205869cc28/package.json#L13-L15) file, but that is pretty obscure unless you know to look for it.

> Is this project not being maintained anymore?

I do not have any current plans for big changes for chessboard.js in the near future, but I have not ruled out working on a v2 completely. Meanwhile, I'm around to watch the repo for small stuff like this :wink: 
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