in your readme, you've suggested to use these launch options "C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\StardewModdingAPI.exe %command%
but there's an easier way like StardewModdingAPI %command%
Somehow if you omit the .exe, Steam tries to figure out the right path, starting from the game's folder.
I've figured it out while trying to have it launch with a relative path, even if it meant to launch through cmd. But luckily, it's not even necessary.

Anyway, I suggest to update the readme so people don't have to figure out their game's path and might end up with mistakes like missing quotes or that like. (btw. the readme is missing quotes or rather the closing quote)

Thanks! Starting with SMAPI 1.0, we'll probably have an install script (#155) that enables Steam support without further user action. That will work by replacing the StardewValley.exe itself (with a backup).