[PATCH v9 14/14] doc/user_manual.md: Document details about deb-src caching

[Vijai Kumar K <Vijaikumar_Kanagarajan@mentor.com>]

Add details about deb-src caching.

Signed-off-by: Vijai Kumar K <Vijaikumar_Kanagarajan@mentor.com>
---
 doc/user_manual.md | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index dd6fc34..8827b29 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -919,3 +919,33 @@ And build the corresponding image target:
 ```
 bitbake mc:qemuarm64-buster:isar-image-base
 ```
+## Cache all upstream Debian source packages in local apt
+
+### Motivation
+
+OSS license compliance: Some licenses require to provide the corresponding sources code,
+other require copyright attributions that may be best provided via the source code. In
+addition, you may want to archive the code locally in order to ensure reproducibility (and
+modifiability) in the future.
+
+Currently the local-apt generated has only Debian binary packages. Extend the local-apt
+to have Debian source packages as well.
+
+### Solution
+
+ - Trigger download of Debian source packages as part of rootfs postprocess.
+
+With the current base-apt implementation, we already cache all the binary packages that
+we download and install onto the target rootfs and buildchroot. This is then used to
+generate a local-apt for offline build.
+
+Use rootfs postprocessing to parse through the the list fo deb files in ${DEDDIR} and
+download the corresponding Debian source file using "apt-get source" command.
+This caches the sources of all the Debian packages that are downloaded and installed onto
+the target rootfs and buildchroots.
+
+By default, the Debian source caching is not enabled.
+To enable it, add the below line to your local.conf file.
+```
+BASE_REPO_FEATURES = "cache-deb-src"
+```
-- 
2.17.1