Document the extra apt repository feature

This MR documents the Extra APT Repository feature with some example
screenshots.

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>

README.md

 # The document is now available [here](https://www.apertis.org/guides/component_guide/)
 
+# Injecting an external APT repository to CI Build environment
+
+The package builds are targetted against a particular release of the OS, making use of the specific versioned libraries and
+dependencies shipped with the OS release.
+
+There are scenarios where a user is required to pull in external dependencies.
+
+* User wants to build a package, which has dependencies on other packages, that are not part of the OS release
+* User wants to build a proprietary package, which is hosted in a separate private repository
+
+
+The CI Build Environment provides a mechanism to inject external APT repository to cover the above mentioned use cases.
+
+There are 3 environment flags that define this behavior.
+
+* EXTRA_REPO_BASE_URL
+* EXTRA_REPO_VERSION
+* EXTRA_REPO_COMPONENTS
+
+`EXTRA_REPO_BASE_URL` defines the APT repository url. It only supports standard http/s based urls, as well as http password
+protected repository urls. Eg. `https://username:password@repositories.apertis.org/apertis/`
+This variable is mandatory.
+
+`EXTRA_REPO_VERSION` defines the APT repository release. An APT repository may host multiple releases, each distinguished by
+its release names. An example release name is: `v2022`.
+This variable is optional. If not specified, the release is determined from the running git branch.
+
+`EXTRA_REPO_COMPONENTS` defines the APT repository components. Multiple repository components may be specified by space.
+Eg. "target sdk development". This variable is mandatory.
+
+
+  > ✍️ Note!
+  >
+  > If the mandatory options are not specified, the CI machinery will print a warning about it and disregard the extra APT repository
+
+
+Below are some screenshots on how these variables and their values are fed into Gitlab CI
+
+![Gitlab CICD](/imgs/env-variables-main.png "Gitlab CICD Overview")
+![EXTRA_REPO_BASE_URL](/imgs/base-url.png "EXTRA_REPO_BASE_URL variable definition")
+![EXTRA_REPO_VERSION](/imgs/repo-version.png "EXTRA_REPO_VERSION variable definition")
+![EXTRA_REPO_COMPONENTS](/imgs/extra-components.png "EXTRA_REPO_COMPONENTS variable definition")
+
 # Updating Existing Components
 
 ## Manually Updating Components from Debian