.gitignore: Ignore .yarn/ (for yarn berry experimental branch, #1618)

Even though we may not ultimately use it:

- It seems to be a project in the midst of a philosophical problem.
- Why give it another name, unless losing track of where it came from
- It seems to decouple architecture (yay) but I'm concerned
  with not adopting an incremental approach to make yarn v1 better.

  Why not decouple yarn v1. Slowly improve that one.
- 99% of the options berry has make no sense to me. Overwhelmed by
  choices.

  Every configuration option is listed, with no help given to making the
  bread and butter scenarios prominent.

  My guess is the settings are to fill niche caches and specialized
  project layouts.

  What would help:

  Show practical examples of what type of projects these configuration
  settings are for. If it's not fitting the scenario - don't even
  mention it.
- Not a fan of how YN0000 shows everywhere.
  yarnpkg/berry#2517 since Feb 25, 2021

  This may have a practical use case for it, e.g. when certain
  operations run in parallel. But this shows even when an operation
  isn't parallel. Eek.
- The underlying feeling I have is berry is optimizing for the maintainers,
  and the user is an afterthought.

  This is direct opposite to what yarn is about. Yarn is about hiding
  complexity, user convenience and happiness, and performance.

  As odd as that may sound - it's important to strike a balance of a
  good product and a good developer experience.
- I still see a lot of potential and benefit in berry

  As an open source maintainer

  The architecture is there, it just needs to get polished to show what
  matters to the user, to convey its value.

.gitignore

@@ -20,6 +20,10 @@ npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
 
+# yarn berry
+.yarn/*
+.pnp.*
+
 # generated files from scripts/github.js
 packages/data/scraped/