scalar: start documenting the command

Let's build up the documentation for the Scalar command along with the
patches that implement its functionality.

Note: To discourage the feature-incomplete documentation from being
mistaken for the complete thing, we do not yet provide any way to build
HTML or manual pages from the text file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/scalar/scalar.txt

@@ -0,0 +1,38 @@
+scalar(1)
+=========
+
+NAME
+----
+scalar - an opinionated repository management tool
+
+SYNOPSIS
+--------
+[verse]
+scalar <command> [<options>]
+
+DESCRIPTION
+-----------
+
+Scalar is an opinionated repository management tool. By creating new
+repositories or registering existing repositories with Scalar, your Git
+experience will speed up. Scalar sets advanced Git config settings,
+maintains your repositories in the background, and helps reduce data sent
+across the network.
+
+An important Scalar concept is the enlistment: this is the top-level directory
+of the project. It usually contains the subdirectory `src/` which is a Git
+worktree. This encourages the separation between tracked files (inside `src/`)
+and untracked files, such as build artifacts (outside `src/`). When registering
+an existing Git worktree with Scalar whose name is not `src`, the enlistment
+will be identical to the worktree.
+
+The `scalar` command implements various subcommands, and different options
+depending on the subcommand.
+
+SEE ALSO
+--------
+linkgit:git-maintenance[1].
+
+Scalar
+---
+Associated with the linkgit:git[1] suite