Document APLcart headers and entry placement in table.tsv

[rodrigogiraoserrao]

Every time I contribute to APLcart I have a very hard time knowing what to add in the CLASS, TYPE, GROUP and CATEGORY columns.
Not only I don't really understand the differences between some of those columns, I never know what are the possible values each column can take.

Another minor issue is that I never know where to place new code entries. At the bottom of the table? Close to similar entries? Semantically similar? Similar in length? (Tried running ReorderFile after making sure Test gives 1, but ReorderFile gave a weird LENGTH ERROR when trying to split the table into its columns.)

It could be interesting/relevant/helpful if a basic wiki page (or similar) were created, documenting this aspect of the table.tsv file.