Add note on composer update dependency mismatch

[Steveb-p]

On branches other that master or version branches (existing in Symfony repository) composer update command will fail, since those cannot be installed.

For example, when I've tried to run tests for Redis Messenger message rejection commit, I've got this error output (trimmed a little, and I was on a specific commit, but it happens on branches as well):

$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - doctrine/orm 2.4.x-dev requires symfony/console ~2.0 -> satisfiable by symfony/console[2.0.4
...
    - don't install symfony/console 2.0.4|remove symfony/symfony dev-b40c1abf9ace9411747a9655dabcb560457bf88d
    - don't install symfony/console 2.0.5|remove symfony/symfony dev-b40c1abf9ace9411747a9655dabcb560457bf88d
    - don't install symfony/console 2.0.6|remove symfony/symfony dev-b40c1abf9ace9411747a9655dabcb560457bf88d
    - don't install symfony/console 2.0.7|remove symfony/symfony dev-b40c1abf9ace9411747a9655dabcb560457bf88d
...

Figured I have to be on one of existing Symfony branches for the install to proceed, and once I've switched to master, I was good to go.

Is the selected doc branch correct in this PR? Is the wording in the tip ok?

[xabbuh]

I would show the usage of the COMPOSER_ROOT_VERSION env var instead like this:

COMPOSER_ROOT_VERSION=5.1.x-dev symfony composer update

[Steveb-p]

I didn't know it works like that actually 😄

[Steveb-p]

@xabbux I changed the tip accordingly. Is the wording ok?

[xabbuh]

I wonder if we shouldn't simply always make use of the env var and simply add a sentence like "replace 5.1 with the Symfony version your patch targets" or so.

[nicolas-grekas]

This PR has been closed because the master has been removed.
Please submit it again against the appropriate branch.