Markdown is used for creating README page automatically in GitHub. ReText is good GUI tool for writing Markdown. You can check the view of Markdown in GitHub with ReText's live preview.
1 Install ReText
Install retext package.
$ sudo apt install -y retext
2 Write Markdown with ReText
Click ReText icon in Desktop menu or run retext command in terminal.
For using live preview, press "Preview" menu, press arrow at the side of "Preview" or press Ctrl + e key. Because live preview can detect file update, you can use your editor like emacs or vim for writing markdown while checking the view of markdown with live preview.