docs: document dcm2nii --dwi (4-D DWI stack + FSL .bval/.bvec)

Author: Fuli Wu

docs/quickstart.md

@@ -78,7 +78,8 @@ $ dcmclient dcm2waveform ecg.dcm out.csv        # ECG/EEG samples → CSV / WAV
 
 ```console
 $ dcmclient dcmconv   --to j2k ct.dcm out.dcm   # transcode (also: rle / jls / htj2k / jxl / …)
-$ dcmclient dcm2nii   ct_series/ ct.nii.gz      # validated NIfTI
+$ dcmclient dcm2nii   ct_series/ ct.nii.gz      # validated 3-D NIfTI
+$ dcmclient dcm2nii   --dwi dti_series/ dwi.nii.gz   # 4-D DWI stack + FSL .bval/.bvec
 $ dcmclient img2dcm   photo.jpg out.dcm         # JPEG → Secondary Capture
 $ dcmclient pdf2dcm   report.pdf out.dcm        # wrap a PDF  ↔  dcm2pdf extracts it back
 $ dcmclient dcm2pdf   report.dcm report.pdf     #   …the reverse: pull the embedded document out