Markdown is a plain-text file format. The extensions
.markdown are just text files written in Markdown syntax. If you have a
Readme.md in your repo, GitHub will show the contents on the home page of your repo. Read the documentation:
You can edit the
Readme.md file in GitHub itself. Click on Readme.md, you will find an edit button. You can preview your changes and even commit them from there.
Since it is a text file, Notepad or Notepad++ (Windows), TextEdit (Mac) or any other text editor can be used to edit and modify it. Specialized editors exist that automatically parse the markdown as you type it and generate a preview, while others apply various syntax coloring and decorations to the displayed text. In both cases though, the saved file is still a readable text file.
If you want to create an
md file with preview and if you prefer not to install any special editors, you can use online editors like dillinger.io and stackedit.io. They provide live preview. You can also export your files to Google Drive or Dropbox.