Add swiftly documentation

Author: plemarquand

userdocs/userdocs.docc/supported-toolchains.md

@@ -16,3 +16,16 @@ 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).
+
+You can choose a swiftly managed toolchain to use from the `> Swift: Select Toolchain` menu.
+
+If you do `swiftly use` on the command line you must restart VS Code or do `> Developer: Reload Window` in order for the VS Code Swift extension to start using the new toolchain.
+
+### `.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).