"To come" is a printing and journalism reference, commonly abbreviated to "TK." It is used to signify that additional material will be added at a later date.
"tk" as an abbreviation may originally have come into use because very few words feature this letter combination. The phrase "to come," by contrast, could potentially be mistaken as a deliberate part of the text.
The Chicago Manual of Style online Q&A on manuscript preparation describes this shorthand as "imprecise", stating, "It’s best to be more straightforward and specific. For example, use bullets or boldface zeros (••• or 000) to stand in for page numbers that cannot be determined until a manuscript is paginated as a book (but see paragraph 2.37 in CMOS). For items like missing figures, describe exactly what’s missing. In electronic environments, you have recourse to comment features—like the syntax of SGML, which allows for descriptive instructions that will not interfere with the final version of a document. Make sure that whatever you do stops the project in its tracks at some point before publication."