changelog: add mention of new 'src snapshot' commands (#891)

Author: bobheadxi

CHANGELOG.md

@@ -24,6 +24,7 @@ All notable changes to `src-cli` are documented in this file.
 ### Added
 
 - Batch specs being run locally with `src batch preview` or `src batch apply` can now be run with the `-run-as-root` flag, which will run all step containers as root instead of the default user for the image. This is off by default. [#886](https://github.com/sourcegraph/src-cli/pull/886)
+- An **experimental** set of `src snapshot` commands for generating and sharing snapshots of Sourcegraph instances, as well as testing Sourcegraph instances restored from snapshots. [#888](https://github.com/sourcegraph/src-cli/pull/888), [#889](https://github.com/sourcegraph/src-cli/pull/889), [#890](https://github.com/sourcegraph/src-cli/pull/890)
 
 ### Changed