Added support for downloading mods from Modrinth (#1577)

README.md

@@ -684,6 +684,20 @@ For example, the following will auto-download the [EssentialsX](https://www.spig
 
     -e SPIGET_RESOURCES=9089,34315
 
+### Auto-download mods from Modrinth
+
+[Modrinth](https://modrinth.com/) is an open source modding platform with a clean, easy to use website for finding [Fabric and Forge mods](https://modrinth.com/mods). At startup, the container will automatically locate and download the newest versions of mod files that correspond to the `TYPE` and `VERSION` in use. Older file versions downloaded previously will automatically be cleaned up.
+
+- **MODRINTH_PROJECTS** : comma separated list of project slugs (short name) or IDs. The project ID can be located in the "Technical information" section. The slug is the part of the page URL that follows `/mod/`:
+  ```
+    https://modrinth.com/mod/fabric-api
+                             ----------
+                              |
+                              +-- project slug
+  ```
+- **MODRINTH_DOWNLOAD_OPTIONAL_DEPENDENCIES**=true : required dependencies of the project will _always_ be downloaded and optional dependencies can also be downloaded by setting this to `true`
+- **MODRINTH_ALLOWED_VERSION_TYPE**=release : the version type is used to determine the newest version to use from each project. The allowed values are `release`, `beta`, `alpha`.
+
 ### Downloadable mod/plugin pack for Forge, Fabric, and Bukkit-like Servers
 
 Like the `WORLD` option above, you can specify the URL or path of a "mod pack"
@@ -899,7 +913,7 @@ Datapacks Json:
   "packs": {
     "survival": [
       "graves",
-      "multiplayer sleep",
+      "multiplayer sleep"
     ],
     "items": ["armored elytra"]
   }