Merge pull request #80 from sourcery-ai/lsp-client-docs

Add docs for writing an LSP client

WritingAnLSPClient.md

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+# Introduction
+Sourcery is a standards compliant Language Server as defined by the [LSP spec](https://microsoft.github.io/language-server-protocol/specifications/specification-current/).
+
+To create a fully working client (code editor extension) you only need to:
+1. Download and unpack the binaries.
+2. Start up the relevant binary for the current OS.
+3. Initialize the Sourcery Language Server with Sourcery specific initialization options.
+4. Follow the standard LSP spec (almost all editors have libraries for doing this).
+5. Tell us about it so we can spread the word!
+
+## Binaries
+The latest binaries are available on the [sourcery-ai/sourcery releases page](https://github.com/sourcery-ai/sourcery/releases/latest). Once downloaded they must be unpacked before they can be run.
+
+The unpacked binaries can be checked on your system by running `path_to_binary/sourcery --verify` which will print out a message similar to:
+```
+sourcery, version 0.7.11
+
+Verifying server availability https://api.sourcery.ai/health - success
+Verifying refactoring functionality - success
+```
+
+## Startup and Initialisation
+The client communicates with The Sourcery Language Server through standard input and output. To run the Sourcery Language Server execute `path_to_binary/sourcery --lsp`.
+
+The Sourcery Server is initialised with the standard [LSP initialize request](https://microsoft.github.io/language-server-protocol/specifications/specification-current/#initialize). It requires the `initializationOptions` param to be populated as shown below and will respond with the list of ServerCapabilities that it supports.
+
+```ts
+interface SourceryInitializationOptions: {
+    /**
+     * The Sourcery token for authenticating the user.
+     * This is retrieved from the Sourcery website and must be
+     * provided by each user. The extension must provide a
+     * configuration option for the user to provide this value.
+     */
+    token: string;
+
+    /**
+     * The extension's name and version as defined by the extension.
+     * e.g. sourcery-ai/vscode 0.7.6
+     */
+    extension_version: string;
+
+    /**
+     * The editor's name and version as defined by the editor.
+     * e.g. vscode 1.44.0
+     */
+    editor_version: string;
+}
+```
+
+Once the Sourcery Language Server is initialized is will act in a fully LSP compliant manner providing refactoring suggestions and hover functionality.
+
+## Let us know!
+if you implement a new client for a editor we'd love to hear about it and are happy to help if you need any assistance. Just drop us a line at hello@sourcery.ai