Hello everyone.
We're happy to announce that Steam Workshop has been implemented and is currently available to the public in it's beta testing stage.
There may also be a couple of minor issues, if you happen to stumble upon any of them, please report them down in the comment section.
Regarding mod creation and uploading:
There is an executable (ContentUploader.exe) in the "\Steam\SteamApps\common\Everlasting Summer\game\mods" folder that allows you to upload mods. You should point out the path to your mod (e.g. "C:\My_cool_mod\", write a name and description and select an image - e.g. "C:\My_cool_mod\image.jpg", all the steps are mandatory!)
All your downloaded workshop items can be found here: "\Steam\SteamApps\workshop\content\331470\"
There is also an official mod available (russian only), which is opensource.
Here's a video guide on using the Content Uploader tool.
Edit: Mod incompatibility issues have been fixed, new patch rolled out.