README: Update installation instructions

Thanks to Dave Steele

README.md

@@ -3,26 +3,19 @@
 Goopg is an extension for the **Chrome** and the **Chromium** browser which
 enables GPG sign and verification in the Gmail web page.
 
-# Installation:
+![preview.png](http://leoiannacone.github.io/goopg/images/preview.png)
+
+## Installation:
 The installation consists in two phases:
  * Install the browser extension from [this link](https://chrome.google.com/webstore/detail/goopg/ifpoaednafmgolabhpjmbimllaoidelg)
  * Install the goopg plugin:
 ```
- sudo add-apt-repository ppa:team-goopg/goopg
- sudo apt-get update
- sudo apt-get install goopg-chromium
+ sudo apt-get install goopg
 ```
- Replace `goopg-chromium` with `goopg-chrome` if you use Chrome.
-
- Eventually, if the repo does not work with you, you can use these packages:
-  * [goopg-chrome_0.3.1~xenial1_all.deb (2.4 KiB)](https://launchpad.net/~team-goopg/+archive/ubuntu/goopg/+files/goopg-chrome_0.3.1~xenial1_all.deb)
-  * [goopg-chromium_0.3.1~xenial1_all.deb (2.4 KiB)](https://launchpad.net/~team-goopg/+archive/ubuntu/goopg/+files/goopg-chromium_0.3.1~xenial1_all.deb)
-  * [goopg-common_0.3.1~xenial1_all.deb (9.9 KiB)](https://launchpad.net/~team-goopg/+archive/ubuntu/goopg/+files/goopg-common_0.3.1~xenial1_all.deb)
-
 
 Open http://gmail.com
 
-# Missing gpg-agent
+### Missing gpg-agent
 
 In case goopg is able to check signatures but unable to sign and send emails,
 chances are your system lacks a properly configured gpg-agent to unlock your
@@ -31,12 +24,12 @@ private key
 In this case, please check [this link](https://wiki.archlinux.org/index.php/GnuPG#gpg-agent)
 and follow the instructions there to enable gpg-agent.
 
-#### If you want help in develop
+## If you want help in develop
 
 If you would like to help in developing, take a look at the [dev-install](doc/dev-install.md)
 and at the [project documentation](doc/project.md).
 
-# Donate
+### Donate
 If you like this project, please consider a donation. See the [homepage](http://leoiannacone.github.io/goopg/)
 for more info.