add software heritage badge

Author: kermitt2

Readme.md

@@ -1,7 +1,8 @@
 # pdfalto
 
-[![License: GPL v2](https://img.shields.io/badge/License-GPL%20v2-blue.svg)](https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html)
 [![Build Status](https://travis-ci.org/kermitt2/pdfalto.svg?branch=master)](https://travis-ci.org/kermitt2/pdfalto)
+[![SWH](https://archive.softwareheritage.org/badge/origin/https://github.com/kermitt2/pdfalto/)](https://archive.softwareheritage.org/browse/origin/https://github.com/kermitt2/pdfalto/)
+[![License: GPL v2](https://img.shields.io/badge/License-GPL%20v2-blue.svg)](https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html)
 
 **pdfalto** is a command line executable for parsing PDF files and producing structured XML representations of the PDF content in [ALTO](https://github.com/kermitt2/pdfalto/blob/master/schema/alto.xsd) format.