You should be able to upload it directly through Discourse (i.e. this forum), there’s an “upload” button right in there within the post formatting toolbar.
If you can’t use the Discourse function to do it , then you can use an upload site such as Dropbox, Google Drive or any number of other ones – there are plenty which are free and easy to use. Whatever you use, you’ll be able to get a “share link” which you can copy/paste here, allowing people to find the page where they’ll be able to download your save.
You may also want to look at uploading the file as a .zip instead of as a regular folder, although honestly the save files aren’t usually large enough to absolutely require that… if you know how to do it already then it’s just convenient for people downloading it (purely because it means a smaller/faster download), but if it’s a small save then I wouldn’t bother with it.
Finally, for the benefit of anyone who might not know where to find their save files: for Steam users, the easiest thing to do is go into your Steam library, right-click Stonehearth, select “properties” and then choose the option “show local files”. Your save folder will be clearly marked, but the saves inside will not, they have numerical identifiers rather than names. You can easily check which is which, though, by simply opening “metadata.txt” and checking the first couple of lines in there for the town’s name. For people who don’t use Steam to play Stonehearth, simply search for the Stonehearth folder and once you find it the process is the same as for Steam users.