docs: add usage.md and /docs folder

.vscodeignore

@@ -4,6 +4,7 @@
 coverage/**
 out/**
 src/**
+usage.md
 .gitignore
 node_modules/**
 **/tsconfig.json

usage.md

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+# Usage
+
+## How to share extensions with code-server
+
+When you install extensions remotely with VSCode, they are stored on the machine
+here:
+
+```shell
+~/.vscode-server/extensions
+```
+
+If you're using code-server and want to use those same extensions, you can use
+the `--extensions-dir` flag like so:
+
+```shell
+code-server --extensions-dir ~/.vscode-server/extensions
+```