add Skeleton width guide to README Example

[kwangminini]

The reason why Skeleton width becomes 0 when the display of the parent element is flex, and a solution have been added to the README Example.

If the explanation is not enough, please give feedback.
Thanks😀

[srmagura]

Hey @kwangminini, thanks for this. I am having some trouble understanding the section you added to the README, and I think there might be some technical inaccuracies in it. Instead of talking about percent widths, I would say that the <Skeleton> has no intrinsic width, so, by default, it will have a width of 0 when used as a child of a flex container.

[kwangminini]

@srmagura Ok, I got it.😃
I fixed it, can you check it?

[srmagura]

Thank you. I also made some further improvements.

By the way, I recommend installing the Prettier VS Code extension so that your code is automatically formatted when you save. In the react-loading-skeleton repository, we also have Prettier set up in a pre-commit hook which you can opt in to by running yarn setup.