Add more documentation on changelogs (#15701)

* Add more documentation on changelogs

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Add description of modes

changelog/README.md

@@ -1,3 +1,56 @@
 # changelog
 
-This folder holds changelog updates from commit 3bc7d15 onwards. See [hashicorp/go-changelog](https://github.com/hashicorp/go-changelog) for full documentation on the supported entries.
+This folder holds changelog updates from commit 3bc7d15 onwards.
+
+Release notes are text files with three lines:
+
+ 1. An opening code block with the `release-note:<MODE>` type annotation.
+
+    For example:
+
+        ```release-note:bug
+
+    Valid modes are:
+
+     - `bug` - Any sort of non-security defect fix. 
+     - `change` - A change in the product that may require action or
+       review by the operator. Examples would be any kind of API change
+       (as opposed to backwards compatible addition), a notable behavior
+       change, or anything that might require attention before updating. Go
+       version changes are also listed here since they can potentially have
+       large, sometimes unknown impacts. (Go updates are a special case, and
+       dep updates in general aren't a `change`). Discussion of any potential
+       `change` items in the pull request to see what other communication
+       might be warranted.
+     - `deprecation` - Announcement of a planned future removal of a
+       feature. Only use this if a deprecation notice also exists [in the
+       docs](https://www.vaultproject.io/docs/deprecation).
+     - `feature` - Large topical additions for a major release. These are
+       rarely in minor releases. Formatting for `feature` entries differs
+       from normal changelog formatting - see the [new features
+       instructions](#new-and-major-features).
+     - `improvement` - Most updates to the product that aren’t `bug`s, but
+       aren't big enough to be a `feature`, will be an `improvement`.
+
+ 2. A component (for example, `secret/pki` or `sdk/framework` or), a colon and a space, and then a one-line description of the change.
+
+ 3. An ending code block.
+
+This should be in a file named after the pull request number (e.g., `12345.txt`).
+
+There are many examples in this folder; check one out if you're stuck!
+
+See [hashicorp/go-changelog](https://github.com/hashicorp/go-changelog) for full documentation on the supported entries.
+
+## New and Major Features
+
+For features we are introducing in a new major release, we prefer a single
+changelog entry representing that feature. This way, it is clear to readers
+what feature is being introduced. You do not need to reference a specific PR,
+and the formatting is slightly different - your changelog file should look
+like:
+
+    changelog/<pr num OR feature name>.txt:
+    ```release-note:feature
+    **Feature Name**: Description of feature - for example "Custom password policies are now supported for all database engines."
+    ```