[Feature] Add legends

[latot]

Hi, I'm using corrplot, which is great to show easy the data, is just there is a lot of data that has too much text to be described in the columns, I think would be great be able to add a legend of acronyms to the right or left of the plot, I was not able to found how to do this from corrplot.

One option is think this as add a legend, but be able to add text to the right or left would do the same trick.

If we can already do this, this issue should be moved to the docs, I have checked the function and the descriptions, sadly I was not able to found how to achieve that.

Thx!