[Buildroot] [PATCH 2/2] docs/manual/adding-packages-cargo.adoc: add instructions for offline vendoring

[William Sherrer <william.sherrer1@gmail.com>]

This commit adds documentation to the cargo package infrastructure for offline vendoring.
It provides an example config for a user for what config file they'll need and where to
install it if working in an offline airgapped environment.

Signed-off-by: William Sherrer <william.sherrer1@gmail.com>
---
 docs/manual/adding-packages-cargo.adoc | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/docs/manual/adding-packages-cargo.adoc b/docs/manual/adding-packages-cargo.adoc
index 4df758537b..f966a1497f 100644
--- a/docs/manual/adding-packages-cargo.adoc
+++ b/docs/manual/adding-packages-cargo.adoc
@@ -54,6 +54,25 @@ It is still possible to define custom build commands or install
 commands (i.e.  with FOO_BUILD_CMDS and FOO_INSTALL_TARGET_CMDS).
 Those will then replace the commands from the cargo infrastructure.
 
+When building custom cargo packages in an offline environment, Buildroot will
+attempt to download your dependencies from crates.io by default. Buildroot builds 
+the package tarball with everything needed to compile the package offline.
+However, to use an offline crates mirror or registry to download dependencies,
+add a +config.toml+ pointing cargo to the offline registry.
+This file should exist in the +output/build/.cargo/+ directory.
+
+Below is an example config using panamax as a mirror.
+
+----
+01: [source.my-mirror]
+02: registry = "http://panamax.internal/crates.io-index"
+03: 
+04: [source.crates-io]
+05: replace-with = "my-mirror"
+----
+
+This config will need to be installed as part of the build process.
+
 ==== +cargo-package+ reference
 
 The main macros for the Cargo package infrastructure are
@@ -96,6 +115,4 @@ the tarball cached in Buildroot's +DL_DIR+, and therefore the hash of
 the package's tarball doesn't only cover the source of the package
 itself, but also covers the sources of the dependencies. Thus, a change
 injected into one of the dependencies will also be discovered by the
-hash check. In addition,  this mechanism allows the build to be
-performed completely offline since cargo will not do any downloads
-during the build. This mechanism is called vendoring the dependencies.
+hash check. This mechanism is called vendoring the dependencies.
-- 
2.43.0

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