README: add clipboard tool dependency (nvim-lua#886)

Fixes: nvim-lua#884
Neovim requires an external tool for proper system clipboard integration.
Some systems install this already by default:
- on Fedora xsel is already installed by default
- on Windows using the choko install the win32yank is alredy installed
This is not installed by default on ubuntu or debian so adding that
to the dependencies list and to the install instructions snippets.

Author: dam9000

README.md

@@ -24,6 +24,7 @@ If you are experiencing issues, please make sure you have the latest versions.
 External Requirements:
 - Basic utils: `git`, `make`, `unzip`, C Compiler (`gcc`)
 - [ripgrep](https://github.com/BurntSushi/ripgrep#installation)
+- Clipboard tool (xclip/xsel/win32yank or other depending on platform)
 - A [Nerd Font](https://www.nerdfonts.com/): optional, provides various icons
   - if you have it set `vim.g.have_nerd_font` in `init.lua` to true
 - Language Setup:
@@ -182,7 +183,7 @@ wsl --install
 wsl
 sudo add-apt-repository ppa:neovim-ppa/unstable -y
 sudo apt update
-sudo apt install make gcc ripgrep unzip neovim
+sudo apt install make gcc ripgrep unzip git xclip neovim
 ```
 </details>
 
@@ -192,14 +193,14 @@ sudo apt install make gcc ripgrep unzip neovim
 ```
 sudo add-apt-repository ppa:neovim-ppa/unstable -y
 sudo apt update
-sudo apt install make gcc ripgrep unzip git neovim
+sudo apt install make gcc ripgrep unzip git xclip neovim
 ```
 </details>
 <details><summary>Debian Install Steps</summary>
 
 ```
 sudo apt update
-sudo apt install make gcc ripgrep unzip git curl
+sudo apt install make gcc ripgrep unzip git xclip curl
 
 # Now we install nvim
 curl -LO https://github.com/neovim/neovim/releases/latest/download/nvim-linux64.tar.gz