doc: update config_file: add postproc section

doc/nabu_config_file.md

@@ -52,6 +52,7 @@ Each section describe a usual processing step. In the current version, the avail
 - `preproc`: phase retrieval, CCD corrections, ...
 - `phase`: phase retrieval
 - `reconstruction`: tomography reconstruction
+- `postproc`: post-processing: histogram
 - `output`: output data description
 - `resources`: computing resources description
 - `about`: extra information about the current configuration file