Add links to sourcekit-lsp and swift-playground-mode

Closes #161

Author: taku0

README.md

@@ -185,8 +185,10 @@ For other commands, run `make help`.
 ## Related projects
 
 - [Official swift-mode.el by Apple](https://github.com/apple/swift/blob/master/utils/swift-mode.el): Seems still in very early stage for now. We cannot contribute to it due to the license incompatibility.
+- [sourcekit-lsp](https://github.com/apple/sourcekit-lsp): Language Server Protocol implementation for Swift and C-based languages.
 - [company-sourcekit](https://github.com/nathankot/company-sourcekit): Completion for Swift projects via SourceKit with the help of SourceKitten.
 - [flycheck-swift](https://github.com/swift-emacs/flycheck-swift): Flycheck extensions for Swift.
+- [swift-playground-mode](https://gitlab.com/michael.sanders/swift-playground-mode): Emacs support for Swift playgrounds.
 
 ## Contributing