Initial diagram and developer doc (#1093)

* Sequence diagram for build process
* Developer overview of build process

Co-authored-by: Kanad Kumar <[email protected]>
Signed-off-by: Vikram Karve <[email protected]>

docs/Overview.md

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+# Developer's Guide to Omnibus
+
+## Overview
+
+Omnibus is a command-line tool that helps you define and create trusted OS native packages for software that you want to distribute. Read more about omnibus [here](https://github.com/chef/omnibus/blob/main/README.md).
+
+## Development setup
+
+To setup omnibus for development-
+1. Fork and clone the [omnibus repository](https://github.com/chef/omnibus) in GitHub.
+1. Ensure you have a recent version of Ruby installed on your system.
+1. Run `bundle install` in the repository directory.
+1. You will now have the `omnibus` command available in the `./bin` directory.
+
+## Execution Flow
+
+When `omnibus build <project-name>` is run, the execution sequence is as illustrated [here](./Sequence%20Diagram%20-%20Omnibus%20Build.pdf). An omnibus project typically includes one or more dependency software definitions. Common reusable software definitions can be referred from the [omnibus-sotware](https://github.com/chef/omnibus-software) repository. The project specific build steps are defined in a counterpart software definition of the same name, in a `build do ... end` block.
+
+A summary of steps run during project build
+1. The client (typically the CLI) loads the project
+1. Project 'load' involves loading all software dependency definitions.
+1. The 'load' step parses necessary metadata to help determine software source, license information and to apply project specific overrides.
+1. Once all software dependencies are successfully loaded for the current OS, the 'build' step commences.
+1. The 'build' step is composed of these sub-steps
+   * Based on software definition parsed in the 'load' step, license check is run to ensure that the build adheres to relevant terms in dependencies used.
+   * After license check is successful, the software source is downloaded or pulled from cache.
+   * Each software dependency is built in the order specified across all the definitions. Pre-built binaries may be pulled from cache.
+   * Apart from Windows, on Linux, macOS and other OSes, a health check is run to verify no external or system libraries are linked.
+   * Once all binaries are ready as part of the payload, the OS native installer package is created and signed.
+   * An optional compression step optimizes and 'beautifies' the installer package.