Add initial README.md for the cyclonedx-lib tooling (#3592)

* Add initial README.md for the cyclonedx-lib tooling

Signed-off-by: Stewart X Addison <sxa@redhat.com>

* placate linter

Signed-off-by: Stewart X Addison <sxa@redhat.com>

---------

Signed-off-by: Stewart X Addison <sxa@redhat.com>

cyclonedx-lib/README.md

@@ -0,0 +1,47 @@
+# CycloneDX support files
+
+## Overview
+
+The Temurin project creates Software Bill of Materials (SBoM) files in the
+[CycloneDX](https://cyclonedx.org) format and we use their tooling for doing
+so. The files in this directory providers the interface between our build
+scripts and the CycloneDX API.
+
+The CycloneDX API which we use is written in java and the documentation for
+the API is available at https://cyclonedx.github.io/cyclonedx-core-java
+
+The reference information for the underlying format is available at https://cyclonedx.org/docs/1.5/json/#formulation_items_bom-ref
+
+## Process description
+
+Assuming `--create-sbom` was added the `BUILD_ARGS`, at the end of each
+build the build will call `generateSBoM` in
+[build.sh](https://github.com/adoptium/temurin-build/blob/master/sbin/build.sh).
+This will locate all of the required information, and add it to the SBoM via
+helper methods in
+[sbom.sh](https://github.com/adoptium/temurin-build/blob/master/sbin/common/sbom.sh).
+These functions invoke our
+[TemurinGenSBOM](https://github.com/adoptium/temurin-build/blob/master/cyclonedx-lib/src/temurin/sbom/TemurinGenSBOM.java)
+java code (this directory) which takes the SBOM file as a parameter as well
+as the action to take on that file and adds a suitable section to the SBOM
+file.
+
+## Adding a new value
+
+Adding a new entry to the SBoM can either be done by re-using an existing
+section, or adding the support for a new section. I'll use the example of a
+formulation section (mostly because the author of this added such a section
+recently)
+
+1. Check that the version of CycloneDX you are using supports the
+    functionality you want.  If not, it will need to be updated. To do this
+    you need to do three steps [Sample PR](https://github.com/adoptium/temurin-build/pull/3558/files):
+    - Update [getDependencies](https://github.com/adoptium/temurin-build/blob/master/cyclonedx-lib/getDependencies) with the new version and SHA
+    - Ensure [build.getDependency](https://ci.adoptium.net/job/build.getDependency/) is run to pick up the new version
+    - Update [build.xml](https://github.com/adoptium/temurin-build/blob/master/cyclonedx-lib/build.xml) with the new SHA to allow it to be downloaded successfully during the build (This change can be made along with the getDependencies one if     you co--ordinate the update appopriately
+2. If the build and java code does not already have support for the CycloneDX functionality that you need the follow ([Sample PR](https://github.com/adoptium/temurin-build/blob/master/cyclonedx-lib/build.xml))
+    - Updates to [cyclonedx-lib/TemurinGenSBOM.java])https://github.com/adoptium/temurin-build/blob/master/sbin/common/sbom.sh) to add a new parameter, a new function to implement it, the call to that function from the `switch` functionality in the `main` function
+    - Update [cyclonedx-lib/build.xml](https://github.com/adoptium/temurin-build/blob/master/cyclonedx-lib/build.xml) to add tests for the new functionality
+    - Add a new function to [sbin/common/sbom.sh](https://github.com/adoptium/temurin-build/blob/master/sbin/common/sbom.sh) to add the fields you need
+    - Updates to [sbin/build.sh](https://github.com/adoptium/temurin-build/blob/master/sbin/build.sh) to invoke the new function(s) in sbom.sh
+3. Run the build with `--create-sbom` in the `BUILD_ARGS` and check that it produces the desired output.