Document availability of self-update command
Document availability of self-update command

The self-update command is only available when running as a PHAR (see
commit 761ad6d171); notably, it is not defined in the Ubuntu package.
composer/composer,localheinz/composer,johnstevenson/composer,nicolas-grekas/composer,floverdevel/composer,stof/composer,stefangr/composer,milleruk/composer,alcohol/composer,naderman/composer,staabm/composer