Readdown: To add .md extension support

It's not bad to read markdown document in terminal, however, it's better if we do have another choice, especially it's free and beautiful

Readdown is a good alternative to read markdown document. Unfortunately, it's a little buggy? in case you open a .md or extension other than markdown/mkdn/text/txt, it just throw out an error.

Workaround:
To add more extensions to be supported in Readdown


> vim /Applications/Readown.app/Contents/Info.plist
# update below section
<key>CFBundleTypeExtensions</key>
<array>
<string>markdown</string>
<string>md</string>
<string>mkdn</string>
<string>text</string>
<string>txt</string>
</array>