Add documentation for custom project roles

doc/custom-project-roles.markdown

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+Custom Project Roles
+====================
+
+You can create custom project roles to apply a set of specific restrictions on the people that belong to this role.
+These custom roles are defined for each project.
+
+A custom role inherits from the project member role. 
+For example, you may want to create a custom role to force someone to follow a process.
+You can have a group of people that are allowed to move tasks only from the column "Work in progress" to the column "Done".
+
+Available restrictions
+----------------------
+
+- Task creation is not permitted
+- Moving tasks only from a given column to another one
+
+Configuration
+-------------
+
+### 1) Create a new custom role
+
+From the project settings, click on the left on the menu **Custom Roles** and at the top of the page click on **Add a new custom role**.
+ 
+![New custom role](screenshots/new_custom_role.png)
+
+Give a name to the role and submit the form.
+
+### 2) Add a restriction to the role
+
+Each role is grouped in a table.
+You can click on the dropdown menu in the table to add a new restriction.
+
+![New column restriction](screenshots/new_restriction.png)
+
+If you choose a column restriction, you just need to select the source and the destination column.
+
+### 3) List of restrictions
+
+![List of restrictions](screenshots/restrictions.png)
+
+### 4) Assign the role to someone
+
+Go to the "permissions" section on the left menu and assign the desired role to the user. 
+
+![Custom project role](screenshots/custom_roles.png)