Document config(include)<- from #210

[CGMossa]

As an outsider, there is no place where the (powerful!) config(include) is documented.
What I know thus far:

• config(include) <- "FILENAME" may be used multiple times, and depending on target in odin_option may refer to "custom_c_files.c", or "custom_r_file.c".
• You can include multiple files.
• Duplicated function names in config(include) are not allowed.

See #210 for reference.

[CGMossa]

Also what is this:

config(base) <- "sir"

source