docs: offline build #49

related neovim/neovim#36610

Author: justinmk

README.md

@@ -5,11 +5,14 @@ This is a *write-only* repo of third-party dependencies for use by the Neovim
 CI build.
 
 - `src/`
-    - cache of dependency sources
-    - auto-updated by [GitHub actions](https://github.com/neovim/deps/blob/master/.github/workflows/nightly.yaml) whenever a dependency is added or bumped in [CMakeLists.txt](https://github.com/neovim/neovim/blob/master/cmake.deps/CMakeLists.txt).
+    - Cache of dependency sources.
+    - https://github.com/neovim/deps/tree/master/src is an auto-updated snapshot
+      of the `.deps/build/src/` produced by the Neovim `make deps` build task.
+      - Hint: Use this with `USE_EXISTING_SRC_DIR=ON` to perform an ["offline" build](https://github.com/neovim/neovim/blob/master/BUILD.md#build-offline).
+    - Auto-updated by [GitHub actions](https://github.com/neovim/deps/blob/master/.github/workflows/nightly.yaml) whenever a dependency is added or bumped in [CMakeLists.txt](https://github.com/neovim/neovim/blob/master/cmake.deps/deps.txt).
     - Used by the [unstable PPA](https://launchpad.net/~neovim-ppa/+archive/ubuntu/unstable). PPA builds aren't allowed network access, so a pre-build step fetches this repo to use as input to the build to ensure we have the latest dependencies.
 - `opt/`
-    - cache for manually-managed dependencies (not auto-updated because the origin is unreliable)
+    - Manually-managed dependencies (not auto-updated because the origin is unreliable).
     - `lua-dev-deps` is a combination of *only* the share files of `busted` and
       `luacheck`. The used versions are specified in the Makefile.