docs(doc): add meta datas and doc about evualute

DOCS.md

@@ -1,3 +1,13 @@
+---
+name: Woodpecker Email
+icon: https://gitnet.fr/deblan/woodpecker-email/raw/branch/develop/logo.svg
+description: plugin to send build status notifications via Email.
+tags: [APT, aptly, publish]
+containerImage: deblan/woodpecker-email
+containerImageUrl: https://hub.docker.com/r/deblan/woodpecker-email
+url: https://gitnet.fr/deblan/woodpecker-email
+---
+
 Use the Email plugin for sending build status notifications via email.
 
 ## Config
@@ -56,6 +66,37 @@ pipeline:
         - octocat@github.com
 ```
 
+### Evaluation
+
+This plugin introduces an optional expression to evaluate (on the fly) whether the mail should be sent or not.
+
+
+```diff
+pipeline:
+  mail:
+    image: deblan/woodpecker-email
+    settings:
+      ...
+    when:
+      - evaluate: 'CI_STEP_STATUS == "failure" || CI_PREV_PIPELINE_STATUS == "failure"'
+```
+
+The problem is that the expression is evaluated before the pipeline is generated. In this case, `CI_STEP_STATUS` does not exist yet and the mail step is ignored unless the previous pipeline failed.
+
+```diff
+pipeline:
+  mail:
+    image: deblan/woodpecker-email
+    settings:
+      ...
++     evaluate: 'CI_STEP_STATUS == "failure" || CI_PREV_PIPELINE_STATUS == "failure"'
+    when:
+-     - evaluate: 'CI_STEP_STATUS == "failure" || CI_PREV_PIPELINE_STATUS == "failure"'
+```
+
+More information about the syntaxe on ([https://woodpecker-ci.org/docs/next/usage/pipeline-syntax#evaluate](https://woodpecker-ci.org/docs/next/usage/pipeline-syntax#evaluate)).
+
+
 ### Custom Templates
 
 In some cases you may want to customize the look and feel of the email message