Improve Makefile documentation for `make schema-init`

[simsong]

Hi! Can you please update the Makefile to indicate what make schema-init does and explain why it moves the git repo to a detached head state?

(base) simsong@nimi dfxml % make schema-init
if [ -z "https://github.com/dfxml-working-group/dfxml_schema.git" ]; then echo 'ERROR:Makefile:Please provide a URL for the Makefile parameter SCHEMA_REPOSITORY_URL.' >&2 ; exit 1 ; fi
if [ ! -d schema ]; then git clone https://github.com/dfxml-working-group/dfxml_schema.git schema ; cd schema ; git checkout $(head -n1 ../dfxml_schema_commit.txt) ; fi
Cloning into 'schema'...
remote: Enumerating objects: 20, done.
remote: Counting objects: 100% (20/20), done.
remote: Compressing objects: 100% (16/16), done.
remote: Total 184 (delta 12), reused 12 (delta 4), pack-reused 164
Receiving objects: 100% (184/184), 59.44 KiB | 3.50 MiB/s, done.
Resolving deltas: 100% (79/79), done.
Note: switching to '1987d6735492617356ad80275ab25608c2767481'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 1987d67 Declare v1.2.0
test -r schema/dfxml.xsd && touch schema/dfxml.xsd