Copy auto installer oneliner to readme

[spookyuser]

Prerequisite

• [x ] Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
• [x ] Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
• [ x] My PR addresses the following pyenv issue (if any)
  • Closes https://github.com/pyenv/pyenv/issues/XXXX

Description

• Here are some details about my PR

I'm always forget where this oneliner is and I feel like having it directly in the readme only makes things easier right?

Tests

• My PR adds the following unit tests (if any)