Update Readme

* Remove unnecessary version number
* Point out different commands for the 'nodejs' package
* Point out that Daily-version of Thunderbird is required for testing the add-on
* Remove redundant 'Hacking'-chapter

Regarding the last point: In my opinion all necessary points are made in the 'INSTALL'-chapter. Furthermore, it is said in 'Hacking' that 'make' should be called in the pdfjs-package. This is confusing as it should be 'node(js) make'
typo here :) I'll merge once you fix this