Bug: Docs reference nonexisting directory

[jrbe228]

This ReadME references a nonexisting distributions folder. See the bullet point

2. Place the STK_Engine_v{version}.zip file in the distributions folder at the same level as this file.

Following the link gives a 404:

Although the directory may exist in a local Git repo, remote repos can't contain an empty directory. Creating a placeholder file inside distributions will fix the issue.