Document installation via Composer

[danepowell]

I think it's easier and more sustainable / generalizable to install Drush Launcher via Composer, rather than manually downloading and placing the Phar file. I'm happen to open a PR if you'd be willing to have this documented in the README.

Basically:

1. Run composer global require drush/drush-launcher
2. Make sure your Composer binary directory is in your path, e.g. on Mac open .bash_profile and add export PATH="$PATH:$HOME/.composer/vendor/bin"

[weitzman]

I have found the opposite. Folks get conflicts when they use global keyword. I think we should skip this. Happy to hear other opinions.

[webflo]

You could add it as third option. We added homebrew a while ago.

@danepowell btw, the phar file has support for self updates.

[micheldrescher]

@weitzman Can you further qualify & quantify this, please?

Looking at http://docs.drush.org/en/master/install/ Drush 9 will support only composer based installations (I am currently on Drush 8.1.15), therefore a well-documented composer based installation would make a lot of sense to me.

In fact, I think it should be included in a way in https://github.com/drupal-composer/drupal-project as it would make a lot of sense for those who run many Drupal 8 dev envs on one system - certainly in smaller dev agencies that implement a drupal ops that doesn't proliferate on a "one container per dev env" policy.

[weitzman]

OK, please add it as last option among the 3.

[derhasi]

I like to use composer globally for these tools too, but as there might be conflicts with other global packages I got used to use composer-bin-plugin.

In that case the process of installing would be:

composer global require bamarni/composer-bin-plugin
composer global bin drush require drush/drush-launcher