Add a document for how releases are built.

doc/How-Releases-Are-Made.md

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+# How O# Releases Are Made
+
+The OmniSharp [release pipeline](https://dev.azure.com/omnisharp/Builds/_build?definitionId=2) runs from the OmniSharp Azure DevOps instance. It is defined in [azure-pipelines.yml](/azure-pipelines.yml).
+
+## Rolling Beta Builds
+
+Merges into the master branch generate an empty draft GitHub release with a beta version tag. The tag created for the release then causes a build that uploads the packages.
+
+```mermaid
+sequenceDiagram
+  autonumber
+  Maintainer ->> GitHub: Merges PR into the `master` branch
+  GitHub --) OmniSharp ADO: Merge to `master` triggers pipeline
+  activate OmniSharp ADO
+  OmniSharp ADO ->> GitHub: Pulls source for omnisharp-roslyn
+  note over OmniSharp ADO: Calculates a build version
+  OmniSharp ADO ->> GitHub: Creates a draft release and `v#35;.#35;.#35;-beta.#35;` tag
+  deactivate OmniSharp ADO
+  GitHub --) OmniSharp ADO: `v*` tag creation triggers pipeline
+  activate OmniSharp ADO
+  OmniSharp ADO ->> GitHub: Pulls source for omnisharp-roslyn
+  note over OmniSharp ADO: Builds packages for various platforms
+  OmniSharp ADO ->> GitHub: Adds packages to release and unmark as draft
+  deactivate OmniSharp ADO
+```
+
+## Official Builds
+
+A maintainer creates an empty draft GitHub release with the appropriate version tag. The tag created for the release then causes a build that uploads the packages.
+
+```mermaid
+sequenceDiagram
+  autonumber
+  Maintainer ->> GitHub: Creates draft release with a`v#35;.#35;.#35;` tag
+  GitHub --) OmniSharp ADO: `v*` tag creation triggers pipeline
+  activate OmniSharp ADO
+  OmniSharp ADO ->> GitHub: Pulls source for omnisharp-roslyn
+  note over OmniSharp ADO: Builds packages for various platforms
+  OmniSharp ADO ->> GitHub: Adds packages to release and unmark as draft
+  deactivate OmniSharp ADO
+```