docs: remove cost, document missing options and improve docs (#653)

source/plugins/posts/README.md

@@ -23,4 +23,7 @@ The recent *posts* plugin displays recent articles you wrote on an external sour
     plugin_posts: yes
     plugin_posts_source: dev.to      # External source
     plugin_people_user: .github.user # Use same username as GitHub login
+    plugin_posts_limit: 4            # Limit to 4 posts
+    plugin_posts_descriptions: yes   # Display article short description (when supported)
+    plugin_posts_covers: yes         # Display article thumbnail (when supported)
 ```

source/plugins/posts/metadata.yml

@@ -1,5 +1,4 @@
 name: "✒️ Recent posts"
-cost: N/A
 category: social
 index: 6
 supports: