DOC: Add population average `FreeSurfer` parcellation info to `README`

[jhlegarreta]

Add population average FreeSurfer parcellation information to README: add short explanation on how it was computed and a link to the corresponding NIfTI file hosted together with the rest of the data in zenodo.

[jhlegarreta]

Waiting to see if we put this into where the rest of the population average data resides in Zenodo, and other computation details if relevant.

[jhlegarreta]

@ljod will tag you once I get information on this from Fan and Yuqian, as I do not have merge rights.

[jhlegarreta]

So the explanation I provided matches what Yuqian provided the only remaining this is eventually making the data available in Zenodo and pointing to it instead of the GitHub file.

[jhlegarreta]

@ljod @zhangfanmark This is ready to be merged.

BTW, I noticed that throughout the README multiple versions/records of the data in zenodo are being used, which may be inaccurate/misleading, as the data in some records may have become deprecated/superseded by newer versions. So using a single link to the latest version of the record, and naming the necessary files for a purpose would avoid such issue. Let me know what you think and if you want me to propose this on a separate PR.