[TASK] Improve visibility of composer notice in install documentation

The notice about running composer install --no-dev is hidden from view.

Move it to the front into an own box to have the same visibility
for all commands that need execution.

Resolves: #72651
Releases: master,7.6
Change-Id: I530a754c319e31285701e1af83ab475aeb59ede6
Reviewed-on: https://review.typo3.org/45829


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Peter Foerger <pfoerger@mac.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>

INSTALL.md

@@ -150,8 +150,13 @@ be upgraded later through the Install Tool:
 /var/www/site/ $ tar xzf typo3_src-7.6.x.tar.gz
 ```
 
-* Important: If you use GIT to fetch the sources, don't forget to run `composer install --no-dev`
-  inside the repository, otherwise your installation won't work!
+* Important: If you use GIT to fetch the sources, don't forget to run the following commands,
+otherwise your installation won't work!
+```
+cd typo3_src
+composer install --no-dev
+cd ..
+```
 
 * Create the symlinks in your Document Root:
 ```