Adding new screenshots and update README

Author: revathskumar

README.md

@@ -3,9 +3,12 @@ Gist Markdown Preview
 
 Preview markdown gist while editing on gist.github.com
 
-![Markdown Preview 1](https://lh4.googleusercontent.com/-HpEwglOsimg/UJAuieD-9LI/AAAAAAAAI_s/XzDpuhAsfi8/s640/rsz_gist1.png)
+* [Firefox](https://addons.mozilla.org/en-US/firefox/addon/gist-markdown-preview/)
+* [Google Chrome](https://chrome.google.com/webstore/detail/gist-markdown-preview/icgfmklfpokeihbnnjjdpnjndickgpkh?hl=en-GB)
 
-![Markdown Preview 1](https://lh4.googleusercontent.com/-Q1j0FHM5Eu0/UJAuijNlhzI/AAAAAAAAI_o/CaHZKVNNCFM/s640/rsz_gist3.png)
+![Markdown Preview 1](/screenshots/gist-create.png)
+
+![Markdown Preview 2](/screenshots/gist-create-preview.png)
 
 License
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