Add docs for Pre-Releases (#2505)

## Issue Addressed

NA

## Proposed Changes

When we released [`v1.4.0-rc.0`](https://github.com/sigp/lighthouse/releases/tag/v1.4.0-rc.0), we added a bunch of text about pre-releases. That information was useful, but somewhat hard to reference in future pre-releases.

This PR adds some docs to the book so whenever we do a pre-release we can point users to these docs for more info.

## Additional Info

NA

book/src/SUMMARY.md

@@ -37,6 +37,7 @@
     * [Advanced Networking](./advanced_networking.md)
     * [Running a Slasher](./slasher.md)
     * [Redundancy](./redundancy.md)
+    * [Pre-Releases](./advanced-pre-releases.md)
 * [Contributing](./contributing.md)
 	* [Development Environment](./setup.md)
 * [FAQs](./faq.md)

book/src/advanced-pre-releases.md

@@ -0,0 +1,40 @@
+# Pre-Releases
+
+[sigp/lighthouse]: https://github.com/sigp/lighthouse
+[sigp/lighthouse/releases/latest]: https://github.com/sigp/lighthouse/releases/latest
+[sigp/lighthouse/releases]: https://github.com/sigp/lighthouse/releases
+[`v1.4.0-rc.0`]: https://github.com/sigp/lighthouse/releases/tag/v1.4.0-rc.0
+[`v1.4.0`]: https://github.com/sigp/lighthouse/releases/tag/v1.4.0
+
+From time-to-time, Lighthouse *pre-releases* will be published on the [sigp/lighthouse] repository.
+These releases have passed the usual automated testing, however the developers would like to see it
+running "in the wild" in a variety of configurations before declaring it an official, stable
+release. Pre-releases are also used by developers to get feedback from users regarding the
+ergonomics of new features or changes.
+
+Github will clearly show such releases as a "Pre-release" and they *will not* show up on
+[sigp/lighthouse/releases/latest]. However, pre-releases *will* show up on the
+[sigp/lighthouse/releases] page, so **please pay attention to avoid the pre-releases when you're
+looking for stable Lighthouse**.
+
+### Examples
+
+[`v1.4.0-rc.0`] has `rc` (release candidate) in the version string and is therefore a pre-release. This
+release is *not* stable and is *not* intended for critical tasks on mainnet (e.g., staking).
+
+However, [`v1.4.0`] is considered stable since it is not marked as a pre-release and does not
+contain `rc` in the version string. This release is intended for use on mainnet.
+
+## When to use a pre-release
+
+Users may wish to try a pre-release for the following reasons:
+
+- To preview new features before they are officially released.
+- To help detect bugs and regressions before they reach production.
+- To provide feedback on annoyances before they make it into a release and become harder to change or revert.
+
+## When *not* to use a pre-release
+
+It is not recommended to use pre-releases for any critical tasks on mainnet (e.g., staking). To test
+critical features, try one of the testnets (e.g., Prater).
+