Add recommendations on how to create a panel

Author: Alberto Pérez García-Plaza

CONTRIBUTING.md

@@ -2,6 +2,22 @@
 
 Please follow these guidelines before sending your contribution.
 
+## Recommended panel contents
+
+You are free of create whatever panel you may want. Following recommendations are based on
+our experience and aim at creating simple panels that don't overwhelm users at first glance:
+
+* On each panel, it should be possible to filter mainly by project, and repository.
+  For those that are related with several data sources, it might useful to filter by
+  data source, or have a panel for each data source.
+* No more than 5 widgets, to avoid overwhelming the user.
+* A markdown widget below with some basic explanations about what we are looking at 
+  (panel definition) and a link to documentation page (the one described at 
+  [Adding or updating documentation section](#adding-or-updating-documentation)).
+
+For instance, [GitHub Issues Efficiency panel](https://chaoss.github.io/grimoirelab-sigils/panels/github-issues-efficiency/)
+was built following these recommendations.
+
 ## Directory structure
 
 There are two important places in Sigils:
@@ -71,6 +87,8 @@ After the execution of kidash you should get at least two files, one per each in
 
 ### Adding or updating documentation
 
+**Sigils documentation is publicly available at https://chaoss.github.io/grimoirelab-sigils**
+
 If you are adding a new panel or updating an existing one, you will be required to document it. Bug fixes usually don't need documentation updates. [Jekyll](https://jekyllrb.com/) is used on top of [GitHub Pages](https://pages.github.com/) to build the web page. For more information on how to configure a local Jekyll instance to work with GitHub Pages see [Jekyll and GitHub Pages section](#jekyll-and-github-pages).
 
 Under `docs/` directory you'll find the following structure: