add SWH identifier and badge

Author: nuest

CITATION.cff

@@ -27,4 +27,4 @@ repository-artifact: https://pypi.org/project/geoextent/0.3.0/
 repository-code: https://github.com/o2r-project/geoextent/releases/tag/v0.3.0
 identifiers:
   - type: "swh"
-    value: "swh:1:rel:TODO"
+    value: "swh:1:rev:41e32824d581e763c58004e498c1bf1f343ef6c7"

README.md

@@ -1,6 +1,6 @@
 # erc-checker
 
-[![Build Status](https://travis-ci.org/o2r-project/erc-checker.svg?branch=master)](https://travis-ci.org/o2r-project/erc-checker) [![Build status](https://ci.appveyor.com/api/projects/status/xbla7j4wpbwauk7p/branch/master?svg=true)](https://ci.appveyor.com/project/nuest/erc-checker-r035a/branch/master) [![npm](https://img.shields.io/npm/v/erc-checker.svg)](https://www.npmjs.com/package/erc-checker) [![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active) [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.2203843.svg)](https://doi.org/10.5281/zenodo.2203843)
+[![Build Status](https://travis-ci.org/o2r-project/erc-checker.svg?branch=master)](https://travis-ci.org/o2r-project/erc-checker) [![Build status](https://ci.appveyor.com/api/projects/status/xbla7j4wpbwauk7p/branch/master?svg=true)](https://ci.appveyor.com/project/nuest/erc-checker-r035a/branch/master) [![npm](https://img.shields.io/npm/v/erc-checker.svg)](https://www.npmjs.com/package/erc-checker) [![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active) [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.2203843.svg)](https://doi.org/10.5281/zenodo.2203843) [![SWH](https://archive.softwareheritage.org/badge/swh:1:dir:a67e6dcbe3a1b5b38aad94fea4095324cd09607a/)](https://archive.softwareheritage.org/swh:1:dir:a67e6dcbe3a1b5b38aad94fea4095324cd09607a;origin=https://github.com/o2r-project/erc-checker.git;visit=swh:1:snp:e3d5cd96bdcee333417ea2192584060bef2a77ac;anchor=swh:1:rev:41e32824d581e763c58004e498c1bf1f343ef6c7;path=//)
 
 A JavaScript library and CLI tool for [ERC](https://github.com/o2r-project/erc-spec) result checking.