fix: use separate logo file for README

The full logo (with "Elixir Companies" text) doesn't work in the
header/footer where it renders at h-8 alongside a text label. Revert
elixircompanies_logo.png to the icon-only version and add a dedicated
elixircompanies_readme_logo.png for the README.

Author: doomspork

README.md

@@ -1,4 +1,4 @@
-![Elixir Companies](public/images/elixircompanies_logo.png)
+![Elixir Companies](public/images/elixircompanies_readme_logo.png)
 
 # Elixir Companies