Add semantics README with links to designs

Signed-off-by: Adolfo García Veytia (Puerco) <[email protected]>

semantics/README.md

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+# SPDX Semantics Reference Archive
+
+This directory holds a number of semantic structure designs representing
+different types of repositories, artifacts and other commonly used software.
+The goal of these designs is to act as a source of reference for tool makers to 
+ensure a unified structure in documents produced by SPDX tools.
+
+## Semantic Reference Designs
+
+The following types of software are represented in this directory:
+
+* OCI Container Images
+  * [MultiArch Index](oci-multiarch-index.md)
+  * [Container Image](oci-image.md)
+  * [Container Layer with Operating System Packages](oci-layer.md)
+* Software Repository
+  * [Universal Model for Code Repository](code-repository.md)
+* Operating System Package
+  * [Universal Model for OS Package](os-package.md)
+  * [rpm Package](os-rpm.md)
+  * [deb Package](os-deb.md)
+  * [apk Package](os-apk.md)
+
+
+## Design Considerations and Objectives
+
+Each of the designs contained here attempts to abstract th object as an SPDX
+package which can be referenced by itself, moved to its own ot to another SBOM
+while allowing for a flexible details.
+
+For example, the SBOM of a container image can be referenced by itself to fully
+describe a container. But that same package can be moved to an SBOM describing 
+a multi-arch index while preserving its structure.
+
+Another example: An RPM package can provide its own SBOM and the package in it can 
+be repurposed by a container layer SBOM to describe all software installed via the 
+OS package manager.