[README/Lexy] Add Lexy CLI tool to the README

[antoniorodr]

I really hope this is not considered spam, and I am sorry if it is or if it considered bold or something. I just wanted to share with you a project I worked on which allows you to have the amazing Learn X in Y minutes experience on your terminal.

I tried to email @adambard a few months ago with this, but I did not get a reply. I hope you are doing fine, Adam!

• [ X] I solemnly swear that this is all original content of which I am the original author
• [ X] Pull request title is prepended with [language/lang-code] (example [python/fr] for Python in French or [java] for multiple Java translations)
• [ X] Pull request touches only one file (or a set of logically related files with similar changes made)
• [ X] Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• [ X] If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md