Create gettingstarted.md

Reinstate `gettingstarted.md` for now as a clone of `README.md`, as older versions of the extension still link to it.

Author: hsenag

docs/gettingstarted.md

@@ -0,0 +1,14 @@
+## Getting Started
+
+To install GitHub Copilot, check out the Getting Started guides:
+- [Visual Studio Code](visualstudiocode/gettingstarted.md#getting-started-with-github-copilot-in-visual-studio-code)
+- [JetBrains](jetbrains/gettingstarted.md#getting-started-with-github-copilot-in-jetbrains)
+- [Neovim](https://github.com/github/copilot.vim#getting-started)
+
+For a tour of GitHub Copilot, visit the homepage at [copilot.github.com](https://copilot.github.com).
+
+## Full documentation
+
+- [Visual Studio Code](visualstudiocode)
+- [JetBrains](jetbrains)
+- [Neovim](https://github.com/github/copilot.vim/blob/release/doc/copilot.txt)