Document which types of content are where, and how to edit them (#175)

This repository is a bit confusing, with the large quantity of
auto-generated don't-edit per-version documentation files in `docs/` and
`versioned_docs/`. This PR augments the README with a short explanation
of what's where and how to make changes.

(It may need changes in future, depending on what happens with #148.)

README.md

@@ -2,6 +2,28 @@
 
 All of the material and stitching for [pantsbuild.org](pantsbuild.org).
 
+## Layout
+
+This repository contains three categories of content. Most of it should be edited in this repository, except for the auto-generated documentation for each version of Pants.
+
+### Evergreen pages
+
+`src/` contains content and configuration that isn't associated with a particular version of Pants. In particular, in `src/pages/` you'll find the definitions of [the front page](https://www.pantsbuild.org) and [community information](https://www.pantsbuild.org/community/getting-help) among others. `src/` also contains helpers like MDX/React components, CSS and JS.
+
+These should be edited directly in this repository.
+
+### Blog
+
+`blog/` contains [the Pants blog](https://www.pantsbuild.org/blog). To write a post, follow the structure of other blog posts.
+
+These should be edited directly in this repository.
+
+### Per-version documentation
+
+`docs/` and `versioned_docs/` contains the documentation for each `2.x` release series of pants. After each release of Pants, the documentation is synced to this repository automatically from https://github.com/pantsbuild/pants: the [`sync_docs.yml`](.github/workflows/sync_docs.yml) workflow manages this, and is triggered by the main repository release process.
+
+This **should not** be edited directly in this repository. Instead, edit in <https://github.com/pantsbuild/pants>. To make changes to documentation for stable release series, edit there and cherry-pick to the appropriate milestone (still in <https://github.com/pantsbuild/pants>), so that it's included in the next release in that series.
+
 ## Development
 
 The docs site is a JS project, so you'll want `nvm` and `yarn` installed.