sparse-checkout.txt: new document with sparse-checkout directions

sparse-checkout.txt: new document with sparse-checkout directions

Once upon a time, Matheus wrote some patches to make
   git grep [--cached | <REVISION>] ...
restrict its output to the sparsity specification when working in a
sparse checkout[1].  That effort got derailed by two things:

  (1) The --sparse-index work just beginning which we wanted to avoid
      creating conflicts for
  (2) Never deciding on flag and config names and planned high level
      behavior for all commands.

More recently, Shaoxuan implemented a more limited form of Matheus'
patches that only affected --cached, using a different flag name,
but also changing the default behavior in line with what Matheus did.
This again highlighted the fact that we never decided on command line
flag names, config option names, and the big picture path forward.

The --sparse-index work has been mostly complete (or at least released
into production even if some small edges remain) for quite some time
now.  We have also had several discussions on flag and config names,
though we never came to solid conclusions.  Stolee once upon a time
suggested putting all these into some document in
Documentation/technical[3], which Victoria recently also requested[4].
I'm behind the times, but here's a patch attempting to finally do that.

[1] https://lore.kernel.org/git/5f3f7ac77039d41d1692ceae4b0c5df3bb45b74a.1612901326.git.matheus.bernardino@usp.br/
    (See his second link in that email in particular)
[2] https://lore.kernel.org/git/20220908001854.206789-2-shaoxuan.yuan02@gmail.com/
[3] https://lore.kernel.org/git/CABPp-BHwNoVnooqDFPAsZxBT9aR5Dwk5D9sDRCvYSb8akxAJgA@mail.gmail.com/
    (Scroll to the very end for the final few paragraphs)
[4] https://lore.kernel.org/git/cafcedba-96a2-cb85-d593-ef47c8c8397c@github.com/

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>