vendor bins documentation

doc/faqs/vendor-bins.md

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+# bin and vendor/bin
+
+## What is a bin?
+
+Any runnable code that a Composer package would like to pass along
+to a user who installs the package should be listed as a bin.
+
+If a package contains other runnable code that is not needed by the
+user (like build or compile scripts) that code need not be listed
+as a bin.
+
+
+## How is it defined?
+
+It is defined by adding the `bin` key to a project's `composer.json`.
+It is specified as an array of files so multiple bins can be added
+for any given project.
+
+~~~json
+{
+    "bin": ["bin/my-script", "bin/my-other-script"]
+}
+~~~
+
+
+## What does defining a bin in composer.json do?
+
+It instructs Composer to install the package's bins to `vendor/bin`
+for any project that **depends** on that project.
+
+This is a convenient way to expose useful runnable code that would
+otherwise be hidden deep in the `vendor/` directory.
+
+
+## What happens when Composer is run on a composer.json that defines bins?
+
+For the bins that a package defines directly, nothing happens.
+
+
+## What happens when Composer is run on a composer.json that has dependencies with bins listed?
+
+Composer looks for the bins defined in all of the dependencies. A
+symlink is created from each dependency's bins to `vendor/bin`.
+
+Say package `my-vendor/project-a` has bins setup like this:
+
+~~~json
+{
+    "name": "my-vendor/project-a",
+    "bin": ["bin/project-a-bin"]
+}
+~~~
+
+Running `composer install` for this `composer.json` will not do
+anything with `bin/project-a-bin`.
+
+Say project `my-vendor/project-b` has requirements setup like this:
+
+~~~json
+{
+    "name": "my-vendor/project-b",
+    "requires": {
+        "my-vendor/project-a": "*"
+    }
+}
+~~~
+
+Running `composer install` for this `composer.json` will look at
+all of project-b's dependencies and install them to `vendor/bin`.
+
+In this case, Composer will create a symlink from
+`vendor/my-vendor/project-a/bin/project-a-bin` to `vendor/bin/project-a`.