Add swiftly documentation

Author: plemarquand

userdocs/userdocs.docc/supported-toolchains.md

@@ -16,3 +16,12 @@ Feature                  | Minimum Toolchain Required
 ------------------------ | ------------------------------------- 
 lldb-dap debugging       | 6.0
 
+## Swiftly Support
+
+The extension supports toolchains managed by [swiftly](https://github.com/swiftlang/swiftly), the Swift toolchain installer and manager. For instructions on installing swiftly see the [installation instructions on Swift.org](https://www.swift.org/install).
+
+### `.swift-version` Support
+
+Swiftly can use a special `.swift-version` file in the root of your package so that you can share your toolchain preference with the rest of your team. The VS Code Swift extension respects this file if it exists and will use the toolchain specified within it to build and test your package.
+
+For more information on the `.swift-version` file see swiftly's documentation on [sharing recommended toolchain versions](https://swiftpackageindex.com/swiftlang/swiftly/main/documentation/swiftlydocs/use-toolchains#Sharing-recommended-toolchain-versions).