State explicitly that our current maximal length of the path in dandi…

…set must not exceed 512

ATM, if my check is right -- maximal length we can encounter is only 192, so we
are quite ok

	dandi@drogon:/mnt/backup/dandi/dandisets$ pwd
	/mnt/backup/dandi/dandisets

	dandi@drogon:/mnt/backup/dandi/dandisets$ declare m=0;for ds in 00*; do git -C $ds ls-tree -r --name-only HEAD; done | awk '{ if (length($0) > max) max = length($0) } END { print max }'
	192

When user manages to get over -- server 500s. Filed
dandi/dandi-archive#1889

docs/about/policies.md

@@ -19,6 +19,7 @@
   [Neuroimaging Data Model](NIDM), and other [BRAIN Initiative](https://braininitiative.nih.gov/)
   standards. We are working with the community to improve these standards and to
   make DANDI archive FAIR.
+- **File name/path limitations:** There is a limit of 512 characters for the full path length within a dandiset.
 - **Volume and size limitations:** There is a limit of 5TB per file. We currently
   accept any size of standardized datasets, as long as you can upload them over
   an HTTPS connection. However, we ask you contact us if you plan to upload more than 10TB of data.