Note about image name and tag sanitization

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

README.md

@@ -31,6 +31,7 @@ ___
   * [`type=raw`](#typeraw)
   * [`type=sha`](#typesha)
 * [Notes](#notes)
+  * [Image name and tag sanitization](#image-name-and-tag-sanitization)
   * [Latest tag](#latest-tag)
   * [Global expressions](#global-expressions)
     * [`{{branch}}`](#branch)
@@ -625,6 +626,22 @@ tags: |
 
 ## Notes
 
+### Image name and tag sanitization
+
+In order to comply with [the specification](https://docs.docker.com/engine/reference/commandline/tag/#extended-description),
+the image name components may contain lowercase letters, digits and separators.
+A separator is defined as a period, one or two underscores, or one or more
+dashes. A name component may not start or end with a separator.
+
+A tag name must be a valid ASCII chars sequences and may contain lowercase and
+uppercase letters, digits, underscores, periods and dashes. A tag name may not
+start with a period or a dash and may contain a maximum of 128 characters.
+
+To ease the integration in your workflow, this action will automatically:
+
+* Lowercase the image name
+* Replace invalid chars sequences with `-` for tags
+
 ### Latest tag
 
 `latest` tag is handled through the [`flavor` input](#flavor-input). It will be generated by default (`auto` mode) for: