README: rework "plugin" section

* replace "exchange" with "gallery"
* mention the most common issues:
    * multigraph
    * magic markers
    * gallery
    * style checks

README.md

@@ -4,10 +4,21 @@ This is the repository for all user contributed stuff
 
 **This is usually where you want to begin your journey.**
 
-Here you'll find all the plugins coming from http://exchange.munin-monitoring.org/. 
+Here you find a plethora of plugins for the most diverse topics. Please take a look and
-That web site is for the time being disabled, new updates are done here.
+refresh/improve existing or propose new plugins.
+
+See the [gallery](http://gallery.munin-monitoring.org/) for a browsable overview of these plugins.
+
+Hints for code submissions:
+
+* add [documentation](http://guide.munin-monitoring.org/en/latest/develop/documenting.html#plugin-documentation) including configuration, author, license and [magic markers](http://guide.munin-monitoring.org/en/latest/architecture/syntax.html#magic-markers)
+* pick a suitable [category](guide.munin-monitoring.org/en/latest/reference/graph-category.html)
+* you should probably use the [multigraph approach](http://guide.munin-monitoring.org/en/latest/plugin/multigraphing.html#plugin-multigraphing) for non-trivial plugins
+* use style check tools for the language of the plugin (e.g. `shellcheck` for shell and `flake8` for Python)
+* bonus: add [example graphs](http://munin-monitoring.org/wiki/PluginGallery#Rulesforplugincontributors) for the [gallery](http://gallery.munin-monitoring.org/)
+
+See the [plugin development documentation](http://guide.munin-monitoring.org/en/latest/develop/plugins/index.html) for more details.
 
-If a dedicated website comes back alive, its plugin backend will be this git repo.
 
 # contrib/templates/ -  3rd-party templates