Added small docu on how to use --inherit_unbiased_template

docs/how_to/reuse_unbiased_template.md

@@ -0,0 +1,22 @@
+# How to re-run registration only on a few failed subjects, or add new subjects to an old RABIES run
+
+The `--inherit_unbiased_template` preprocessing parameter was designed for cases where a subset of scans failed registration, 
+or new scans need to be added to a study, without re-running the pipeline for all previously well-registered scans.
+Specifically, this parameter allows to recycle the unbiased study template generated by an old RABIES run, and register each
+scan directly to this template and instead of re-generating a new one from the input data (which would require re-running the 
+commonspace registration across the entire dataset).
+
+To use this parameter, you can point `--inherit_unbiased_template` to the path of a previously completed RABIES run, and 
+combine this with `--inclusion_ids` to only preprocess a selected set of scans within the BIDs directory (if it is shared
+with the rest of the study dataset):
+```sh
+rabies --inclusion_ids /full_path/bids_inputs/.../selected_scan1.nii.gz /full_path/bids_inputs/.../selected_scan2.nii.gz \
+preprocess bids_inputs/ preprocess_outputs_selected_scans_only/ \
+--inherit_unbiased_template /path_to_my_old_RABIES_preprocess_output
+```
+Importantly, you will note that you must provide some new output directory for this independent RABIES run, and it is considered independent
+for downstream RABIES stages, as there is currently no way to fully merge together two different RABIES runs.
+
+Also note that by selecting this option, the following preprocessing parameters will be overriden to enforce consistency with 
+the previous run: `--anatomical_resampling`, `--commonspace_reg`, `--anat_template`, `--brain_mask`, `--WM_mask`, `--CSF_mask`, `--vascular_mask`. 
+