Move English documentation to folder en_US

doc/en_US/project-permissions.markdown

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+Project Permissions
+===================
+
+Each project is isolated from other projects.
+Project access must be allowed by the project owner.
+
+Each user and each group can have a different role assigned.
+There are 3 types of [roles for projects](roles.markdown):
+
+- Project Manager
+- Project Member
+- Project Viewer
+
+Only administrators have access to everything.
+
+Role assignments are visible in **Project Settings > Permissions**:
+
+![Project Permissions](screenshots/project-permissions.png)
+
+If you choose to allow everybody, all Kanboard users will be considered 'Project Member'.
+With this setting, role management will have no effect. Permission per user or per group cannot be applied.
+
+Private projects cannot define permissions.