Add descriptive blurb into README.md

[jGaboardi]

• Add descriptive blurb into README.md
• Maybe adapt from nhgisxwalk.GeoCrossWalk:

---

Generate a temporal crosswalk for census geography data built from the smallest intersecting units (atoms). Each row in a crosswalk represents a single atom, and comprised of a source ID (geo+year+gj), and target ID (geo+year+gj), and at least one column of weights. An example of a source ID is bgp1990gj (block group parts from 1990) and an example of a target ID is trt1990gj (tracts from 2010) — see []() for the nhgis_bgp1990gj_to_trt1990gj crosswalk extract of Delaware. The weights are the interpolated proportions of source attributes that are are calculated as being within the target units. For a description of the algorithmic workflow see the General Crosswalk Construction Framework. Data from 1990 poses specific problem due to the US Census Bureau not explicitly including blocks with no population/housing units in the summary files (SF1). For a description of the algorithmic workflow in the 1990 "no data" scenarios see Handling 1990 No-Data Blocks in Crosswalks. For more information of the base crosswalks see their technical details. For further description see Schroeder (2007).

Currently supported crosswalks include:

source	target
1990 block group parts	2010 tracts
2000 block group parts	2010 tracts
1990 block group parts	2010 block groups
2000 block group parts	2010 groups

Planned supported crosswalks include:

source	target
1990 block group parts	2012 tracts
2000 block group parts	2012 tracts
1990 block group parts	2012 block groups
2000 block group parts	2012 groups

• Schroeder, J. P. 2007. Target-density weighting interpolation and uncertainty evaluation for temporal analysis of census data. Geographical Analysis 39 (3):311–335.

[jGaboardi]

46