Add Flatpak installation instructions to README.md (#846)

Tested with https://flathub.org/apps/details/io.neovim.nvim

README.md

@@ -56,13 +56,20 @@ $uri = 'https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
 
 #### Neovim
 
-###### Unix
+###### Unix, Linux
 
 ```sh
 curl -fLo ~/.local/share/nvim/site/autoload/plug.vim --create-dirs \
     https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
 ```
 
+###### Linux (Flatpak)
+
+```sh
+curl -fLo ~/.var/app/io.neovim.nvim/data/nvim/site/autoload/plug.vim \
+    https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
+```
+
 ###### Windows (PowerShell)
 
 ```powershell