minor #12579 Add Composer installation before step 1 for Symfony Flex (alanpoulain)

This PR was merged into the 3.4 branch.

Discussion
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Add Composer installation before step 1 for Symfony Flex

Because Symfony Flex (in the next branches) needs Composer to be installed globally too.

Commits
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6e8d393 Add Composer installation before step 1 for Symfony Flex

Author: wouterj

bundles/best_practices.rst

@@ -203,6 +203,10 @@ following standardized instructions in your ``README.md`` file.
         Installation
         ============
 
+        Make sure Composer is installed globally, as explained in the
+        [installation chapter](https://getcomposer.org/doc/00-intro.md)
+        of the Composer documentation.
+
         Step 1: Download the Bundle
         ---------------------------
 
@@ -213,10 +217,6 @@ following standardized instructions in your ``README.md`` file.
         $ composer require <package-name>
         ```
 
-        This command requires you to have Composer installed globally, as explained
-        in the [installation chapter](https://getcomposer.org/doc/00-intro.md)
-        of the Composer documentation.
-
         Step 2: Enable the Bundle
         -------------------------
 
@@ -248,6 +248,9 @@ following standardized instructions in your ``README.md`` file.
         Installation
         ============
 
+        Make sure Composer is installed globally, as explained in the
+        `installation chapter`_ of the Composer documentation.
+
         Step 1: Download the Bundle
         ---------------------------
 
@@ -258,9 +261,6 @@ following standardized instructions in your ``README.md`` file.
 
             $ composer require <package-name>
 
-        This command requires you to have Composer installed globally, as explained
-        in the `installation chapter`_ of the Composer documentation.
-
         Step 2: Enable the Bundle
         -------------------------