doc: add how to install section

Author: Sébastien HOUZÉ

README.md

@@ -33,3 +33,18 @@ github deployment_status create $DEPLOYMENT_ID in_progress
 # Create a production deployment and corresponding status, short syntax
 github ds c $(github d c -e production -t deploy:migration) in_progress
 ```
+
+## How to install it?
+
+```shell
+go get github.com/inextensodigital/github
+```
+
+or... (feel free to replace `linux` by either `windows` or `darwin`)
+
+```shell
+curl -s https://api.github.com/repos/inextensodigital/github/releases/latest | \
+  jq -r '.assets[] | select(.name | contains("linux-amd64")) | .browser_download_url' | \
+  grep -v sha256 | \
+  wget -qi - -O github && sudo chmod +x github
+```