add doc page for env

docs/30_Configuration/60_File_env.md

+If a file `env` exists in the folder `./jobs/` it will be sourced by the backup scripts.
+Here you can set custom environment variables.
+
+A common usage is extending the variable PATH to execute binaries that are not located in any directory.
+
+Example:
+
+This might be helpful on MS Windows. If you put the restic binary "somewhere" eg. c:\portable\restic\restic.exe then you can add the path to the env file:
+
+```bash
+PATH=/c/portable/restic/:$PATH
+```