Issue #3029769 by jhodgdon, hansfn: Add instructions for converting tarball download to Composer

sub-directory; you'll also get some extra tools such as Drush and Drupal Console

in the _mydir/vendor_ directory.

===== Converting a previously-downloaded site to use Composer

If you previously downloaded the core software without using Composer, you may

find later that you want or need to use Composer to manage and update

dependencies. To convert your existing site to using Composer, enter the

following commands at the root of your site:

----

composer global require grasmash/composerize-drupal

composer composerize-drupal --composer-root=. --drupal-root=.

----

Note that this will also make modules, themes, and profiles that you previously

downloaded without using Composer available to Composer, if they were placed in

the _modules/contrib_, _themes/contrib_, and _profiles/contrib_ directories. If

you placed them in another location, you might consider deleting

previously-downloaded modules and themes, and then following the instructions

below to re-download them using Composer.

===== Using Composer to download a module or theme

Follow these steps if you have already downloaded the core software, and you

want to use Composer to add a contributed module or theme with its

dependencies.

Follow these steps if you are already using Composer to manage the core

software, and you want to use Composer to add a contributed module or theme with

its dependencies.

. Each time you want to add a contributed module or theme, determine the

project's short name. This is the last part of the URL of the project page; for

* https://www.drupal.org/docs/develop/using-composer/using-composer-to-manage-drupal-site-dependencies["Using Composer to manage Drupal site dependencies"]

* https://www.drupal.org/docs/8/update/update-core-via-composer["Update core via Composer"]

* https://github.com/grasmash/composerize-drupal[Composerize Drupal project for converting a previously-downloaded site to using Composer]

*Attributions*

  https://www.drupal.org/docs/develop/local-server-setup[Drupal.org's Local server setup guide]

  for possible options.

Use Composer::

  If you plan to use the Drush tool (see <<install-tools>>), or if you are

  building a site that might use modules with complicated dependencies, you

  should use Composer to download the core software, because Composer will

  manage the dependencies properly. See <<install-composer>> for instructions.

Download manually from the web site::

  If you plan to build a site without add-on modules that have complicated

  dependencies, you can download the core software, or a distribution that

  contains the core software and additional modules or themes, from the web

  site. See <<install-manual>> for instructions.

Use Composer::

  If you plan to use the Drush tool (see <<install-tools>>), or if you are

  building a site that might use modules with complicated dependencies, you

  should use Composer to download the core software, because Composer will

  manage the dependencies properly. If you start your site by downloading

  manually, however, you can convert to using Composer to manage dependencies

  later. See <<install-composer>> for downloading and conversion instructions.

==== What happens when I install the core software?

_Installing_ the core software means setting up some database tables,