Update readme with document families and instructions for getting unique text blocks (#2)

- Update readme with document families and instructions for getting unique text blocks (25b40956e2e2e7cd82aaf2c88bc1a95a03f83b5f)

README.md

@@ -58,6 +58,19 @@ We currently only parse text out of PDFs. Any non-PDF file will only be referenc
 
 We have yet to process approximately 150 documents of the 1700 documents due to formatting issues. We are working on resolving this issue as soon as possible. [See the document list here](https://labs.climatepolicyradar.org/global-stocktake/UNPROCESSED_DOCUMENTS.html).
 
+## Data model
+
+This dataset contains individual documents that are grouped into 'document families'.
+
+The way to think of is as follows:
+
+  * Each row in the dataset is a physical document. A physical document is a single document, in any format.
+  * All physical documents belong to document families. A document family is one or more physical documents, centred around a main document, which jointly contain all relevant information about the main document. For example, where a document has a translation, amendments or annexes, those files are stored together as a family.
+
+### Getting unique text blocks
+
+> TODO
+
 ## Field descriptions
 
 - `author`: document author (str)
@@ -67,6 +80,8 @@ We have yet to process approximately 150 documents of the 1700 documents due to
 - `date`: publication date of the document
 - `document_content_type`: file type. We have only parsed text from PDFs.
 - `document_id`: unique identifier for a document
+- `document_family_id`: see *data model* section above
+- `document_family_slug`: see *data model* section above
 - `document_md5_sum`: md5sum of the document's content
 - `document_name`: document title
 - `document_source_url`: URL for document
@@ -85,7 +100,7 @@ We have yet to process approximately 150 documents of the 1700 documents due to
 ## Known issues
 
 * Author names are sometimes corrupted
-* Text blocks in non-English languages are currently missing or marked as `nan`
+* Text block languages are sometimes missing or marked as `nan`
 
 ## Usage in Python