command-list.txt: documentation and guide line

This is intended to help anybody who needs to update command-list.txt.
It gives a brief introduction of all attributes a command can take.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

command-list.txt

@@ -1,3 +1,48 @@
+# Command classification list
+# ---------------------------
+# All supported commands, builtin or external, must be described in
+# here. This info is used to list commands in various places. Each
+# command is on one line followed by one or more attributes.
+#
+# The first attribute group is mandatory and indicates the command
+# type. This group includes:
+#
+#   mainporcelain
+#   ancillarymanipulators
+#   ancillaryinterrogators
+#   foreignscminterface
+#   plumbingmanipulators
+#   plumbinginterrogators
+#   synchingrepositories
+#   synchelpers
+#   purehelpers
+#
+# The type names are self explanatory. But if you want to see what
+# command belongs to what group to get a better picture, have a look
+# at "git" man page, "GIT COMMANDS" section.
+#
+# Commands of type mainporcelain can also optionally have one of these
+# attributes:
+#
+#   init
+#   worktree
+#   info
+#   history
+#   remote
+#
+# These commands are considered "common" and will show up in "git
+# help" output in groups. Uncommon porcelain commands must not
+# specify any of these attributes.
+#
+# "complete" attribute is used to mark that the command should be
+# completable by git-completion.bash. Note that by default,
+# mainporcelain commands are completable so you don't need this
+# attribute.
+#
+# As part of the Git man page list, the man(5/7) guides are also
+# specified here, which can only have "guide" attribute and nothing
+# else.
+#
 ### command list (do not change this line, also do not change alignment)
 # command name                          category [category] [category]
 git-add                                 mainporcelain           worktree