Add nls readme

lsp/README.md

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+# Nickel Language Server
+
+NLS is a language server for the nickel programming language.  Making use of the interpreter libraries directly, allows nls to offer error messages, type hints, and autocompletion right in your favorite lsp-enabled editor.
+
+## Server
+  
+To be editor independent `nls` is made up of two components the server and possible lsp-client configurations that talk to the server directly.
+Hence, the server has to be installed separately to the client configuration. 
+
+### Installation
+
+The easiest way to install `nls` is using nix. `nls` is installed in the same installation as `nickel`:
+
+
+To enter a shell with `nickel` and `nls` run
+
+```
+nix shell github:tweag/nickel
+```
+
+## Client
+
+Different clients have varying ways to install lsp clients:
+
+### VS Code
+
+Until nls is available through the vscode marketplace, it can be built using nix:
+
+Either using the command line
+
+```
+code --install-extension $(nix build ./\#vscodeExtension --no-link --json | jq ".[0].outputs.vsix")
+```
+
+or in two steps, by first building the extension
+
+```
+nix build github:tweag/nickel#vscodeExtension.vsix
+```
+
+then installing it using `Extension: Install from VSIX` in the vscode command palette and choosing `./result-vsix/nls-client.vsix`.
+
+#### Configuration
+
+The VS Code extension offers three configuration options: 
+
+- `"nls.server.path"`: Path to nickel language server
+- `"nls.server.trace"`: "Enables performance tracing to the given file"
+- `"nls.server.debugLog"`: "Logs the communication between VS Code and the language server."