docs: fix tags documentation

Author: sloria

README.md

@@ -14,7 +14,7 @@ It's very opinionated out the box (as software should be!) but allows you to cus
 ## Inputs
 
 - `context`: The build's context, specifying the set of files located at the provided PATH or URL. It is required to point to your application source code.
-- `tags`: A comma-separated list of tags to apply to the built image. This field is required.
+- `tags`: A comma-separated list of tags to apply to the built image. Defaults to unix timestamp, git SHA, and `latest`.
 - `labels`: An optional, comma-separated list of metadata labels to add to the image.
 - `platforms`: An optional, comma-separated list of target platforms for the build.
 - `pkgs`: Optional additional Nix packages to install in the environment.
@@ -103,4 +103,4 @@ Some things to keep in mind:
 
 * `push` is required when building for multiple architectures.
 * For each platform, an auto-generated tag is generated and pushed.
-* There are some [TODOs](/TODO) that I won't get to until I need them.
+* There are some [TODOs](/TODO) that I won't get to until I need them.