add known issues doc for backend

backend/README.md

 
 In Tracim, only some user can access to some informations, this is also true in
 Tracim REST API. you can check the [roles documentation](doc/roles.md) to check
-what a specific user can do.
+what a specific user can do.
+
+# Known issues
+
+see [here](doc/known_issues.md)

backend/doc/known_issues.md

+# Known issue with Tracim Backend
+
+## Uwsgi
+
+### plaster.exceptions.LoaderNotFound
+
+If you obtain error with :
+
+
+    plaster.exceptions.LoaderNotFound: Could not find a matching loader for the scheme "file+ini"".
+
+
+you most probably don't set correctly TRACIM_CONF_PATH or TRACIM_WEBDAV_CONF_PATH.
+You have to set a correct absolute path.
+be careful for uwsgi ini file :
+
+
+    # incorrect
+    env = TRACIM_CONF_PATH="/home/me/tracim_v2/backend/development.ini"
+
+    # correct
+    env = TRACIM_CONF_PATH=/home/me/tracim_v2/backend/development.ini
+