Add usage section to README.me with link to docs (#1950)

Upon looking for basic usage commands I discovered that there is no
mention of this in the official README.md, just an indirect link to
the installation guide section of the official documentation. I
therefore propose to add a separate usage section, and/or a
documentation section with url to the official documentation for
further reading.

README.md

@@ -36,6 +36,10 @@
 
 Read the [Installation Guide](https://yarnpkg.com/en/docs/install) on our website for detailed instructions on how to install Yarn.
 
+## Using Yarn
+
+Read the [Usage Guide](https://yarnpkg.com/en/docs/usage) on our website for detailed instructions on how to use Yarn.
+
 ## Contributing to Yarn
 
 Contributions are always welcome, no matter how large or small. Before contributing, please read the [code of conduct](CODE_OF_CONDUCT.md).