Updated symlink-project command description in README.

[markdorison]

The Issue

The README says the following about the symlink-project command:

ddev symlink-project. This symlinks the top level files of your project into web/modules/custom so that Drupal finds your module. This command runs automatically on every ddev start. See codebase image below.

This can cause some confusion as it will not run automatically if composer install/update has not already been run so that Composer can generate the vendor/autoload.php file.

How This PR Solves The Issue

Updates README to clarify that this command will not run successfully if vendor/autoload.php has not already been generated.

Manual Testing Instructions

N/A

Automated Testing Overview

N/A

Related Issue Link(s)

Resolves #66

Release/Deployment Notes

N/A