Edits to the BGP crosswalk README

[j-p-schroeder]

• General revisions and reorg, aiming to minimize repetition and maintain simple, clearly defined terminology
• Corrected some character lengths for the geographic codes
• Deleted references to NHGIS data file field names (e.g., STATEA) because those fields (counterintuitively) provide FIPS codes, not NHGIS codes, so to explain these references would require some extra notes, and that doesn't seem worthwhile!
• Deleted URLs to github... Sorry to drop the direct links to the useful documentation there, but we plan to move repository from jGaboardi to IPUMS, and I'd rather not have to update these URLs again when we do that (or worse: forget to update them). I plan to include links on our website, which will be easier to keep updated.

[jGaboardi]

This is failing simply due to the name change of the BGP README file (see here):

247 =========================== short test summary info ============================
248 FAILED nhgisxwalk/tests/test_nhgisxwalk.py::Test_upper_level_functions::test_generate_data_product
249 FAILED nhgisxwalk/tests/test_nhgisxwalk.py::Test_upper_level_functions::test_prepare_data_product
250 FAILED nhgisxwalk/tests/test_nhgisxwalk.py::Test_remove_generated_data::test_remove_generated_data
251 ======================== 3 failed, 49 passed in 19.69s =========================

This name in line 38 should be updated with newly proposed name. Currently:

# special README.txt name for block group parts
BGP_README = "nhgis_bgp"

New version:

# special README.txt name for block group parts
BGP_README = "nhgis_bgp_crosswalk"

I'll go ahead and make that change in this PR, also I'll give you (@j-p-schroeder) admin access to this repo (although it will probably be transferred to IPUMS relatively soon (see #70).