Add new option --sparse to git add help output

Author: petk

book/01-introduction/sections/help.asc

@@ -43,6 +43,7 @@ usage: git add [<options>] [--] <pathspec>...
     --refresh                   don't add, only refresh the index
     --ignore-errors             just skip files which cannot be added because of errors
     --ignore-missing            check if - even missing - files are ignored in dry run
+    --sparse                    allow updating entries outside of the sparse-checkout cone
     --chmod (+|-)x              override the executable bit of the listed files
     --pathspec-from-file <file> read pathspec from file
     --pathspec-file-nul         with --pathspec-from-file, pathspec elements are separated with NUL character