Make documentation for sc13dg_indexes

[bdcallen]

@iangow This issue is a follow-on from #69. I currently have a program, process_sc13dg_indexes.py, running and making the table edgar.sc13dg_indexes. The general idea of this table is that we have a table which indexes key parts of each document and other fields which relate to their style (particularly if they deviate from the usual style of the document). The next idea would be to have a second program which takes data from this table, segments the different sections of the document, and then makes tables tabulating the data in these sections. I feel this table would be good to keep, as it can be used to identify troublesome cases (in a sense, it is somewhat analogous to edgar.forms345_xml_process_table for the forms 3, 4 and 5 project). This issue is for the documentation of this table, and the functions in the program which create it, so that people can understand the table.