Add about the need to bidsignore dandiset.yaml

Author: yarikoptic

docs/user-guide-sharing/validating-files.md

@@ -45,4 +45,11 @@ Passing all of these levels of validation can sometimes be tricky. If you have a
 
 ## Validating BIDS Files
 
-Once your files are converted to the BIDS standard, perform validation of the files using the [BIDS validator](https://bids.neuroimaging.io/tools/validator.html).
+Once your files are converted to the BIDS standard,
+
+  - ensure that you have `.bidsignore` file containing `dandiset.yaml`, since that DANDI-specific file is not part of BIDS specification;
+  - perform validation of the files ideally using both
+    - the stock [BIDS validator](https://bids.neuroimaging.io/tools/validator.html)
+    - and/or [dandi validate](https://dandi.readthedocs.io/en/latest/cmdline/validate.html) since it would also validate using other helper tools such as [NWB inspector](https://nwb.org/tools/core/nwbinspector/) if you have NWBs, to ensure that your data is compliant to all used standards.
+
+Note that [dandi upload](https://dandi.readthedocs.io/en/latest/cmdline/upload.html) will also perform validation during upload.