Add docs

Signed-off-by: Charlie Egan <charlie@styra.com>

docs/remote-features.md

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+# Remote Features
+
+This page outlines the features of Regal that need internet access to function.
+
+## Checking for Updates
+
+Regal will check for updates on startup. If a new version is available,
+Regal will notify you by writing a message in stderr.
+
+An example of such a message is:
+
+```
+A new version of Regal is available (v0.23.1). You are running v0.23.0.
+See https://github.com/StyraInc/regal/releases/tag/v0.23.1 for the latest release.
+```
+
+This message is based on the local version set in the Regal binary, and **no
+user data is sent** to GitHub where the releases are hosted.
+
+This same function will also write to the file at: `$HOME/.config/regal/latest_version.json`,
+this is used as a cache of the latest version to avoid consuming excessive
+GitHub API rate limits when using Regal.
+
+This functionality can be disabled in two ways:
+
+* Using `.regal/config.yaml`: set `features.remote.check-version` to `false`.
+* Using an environment variable: set `REGAL_DISABLE_CHECK_VERSION` to `true`.