Add instructions to install dev version (#28)

* Markdownlint +
added instructions to permanently install
the dev version of the extension.

* Update README.md

It's not the root directory for the repo! That's the lsp.
It is the `addons/vscode` directory!!

Co-authored-by: Andi Péter <andipeti2000@gmail.com>

* Update README.md

Co-authored-by: Nathan Varner <17197562+nvarner@users.noreply.github.com>

* Suggestions

* markdownlint

---------

Co-authored-by: Andi Péter <andipeti2000@gmail.com>
Co-authored-by: Nathan Varner <17197562+nvarner@users.noreply.github.com>

README.md

@@ -3,45 +3,56 @@
 A brand-new language server for [Typst](https://typst.app/).
 
 ## Features
+
 - Syntax highlighting, error reporting, code completion, and function signature
     help
 - Compiles to PDF on save (configurable to as-you-type, or can be disabled)
 
 This repo consists of:
+
 - an LSP server, written in Rust
 - [a corresponding VS Code(ium) extension](https://github.com/nvarner/typst-lsp/tree/master/addons/vscode).
 The extension is available on the [VS Code Marketplace](https://marketplace.visualstudio.com/items?itemName=nvarner.typst-lsp)
 and [OpenVSX](https://open-vsx.org/extension/nvarner/typst-lsp).
 
 ## Near future goals
+
 - Improved preview (e.g. built-in PDF viewer, render to image for speed)
 - Support for more editors
 
 ## Development guide
 
 ### Prerequisites
+
 Install:
+
 - [Rust](https://www.rust-lang.org/) for the LSP itself
 - [Rust Analyzer](https://rust-analyzer.github.io/) an extension for Rust LSP for VS Code
 - [node](https://nodejs.org/en) for the VS Code extension; it may be easiest to
     install via [fnm](https://github.com/Schniz/fnm)
 
 ### First time setup
+
 1. Clone this repository locally
 2. Open it in VS Code; it's needed to run the extension
 3. In the `addons/vscode` subdirectory:
     1. Run `npm install` to install extension dependencies
     2. Run `npm run compile` to build the extension
 4. Run through the development cycle once to initialize and test everything
+5. (Optional: install the dev version of the extension): Press <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>P</kbd>,
+and choose `Developer: Install Extension from Location...` and choose
+the directory for the extension, `addons/vscode/`. There will not be any messages, but
+the extension can be found in the Extensions `@installed` list.
 
 ### Development cycle
+
 1. Make any changes
 2. Run `cargo install --path .`; at present, the VS Code extension just invokes
     the `typst-lsp` command to start the LSP, and this command will compile and
     replace that binary with the latest version
     - If modifying the extension, keep `npm run watch` running, or `npm run compile`
         after changes
-3. Press `ctrl+F5` to launch the "Extension Development Host"; if it's already
+3. Press <kbd>Ctrl</kbd>+<kbd>F5</kbd> to launch the "Extension Development Host"; if it's already
     running, invoke "Developer: Reload Window" from the command palette in the
     Extension Development Host
 4. Within the Extension Development Host, the extension will be active and ready