Document configuration (#32)

Author: ssbarnea

README.md

@@ -64,6 +64,30 @@ Commands:
 There is also an experimental `grib` command line for quering bugs (issues),
 which has almost identical options.
 
+## Configuring
+
+Unless you already have a GerTTY config, here is how to configure the tool:
+
+```yaml
+# ~/.config/gri/gri.yaml
+servers:
+- name: opendev  # server 0, select it with '-s 0'
+  url: https://review.opendev.org/
+- name: rdo  # server 0, select it with '-s 1'
+  url: https://review.rdoproject.org/r/
+  auth-type: basic  # needed only for old gerrit versions
+```
+
+You may be surprised to observe that the credentials are not stored inside
+the same file. That is by design and the tool will load them from `~/.netrc`
+file, which is also the standard way to place network related credentials.
+
+```ini
+# ~/.netrc
+# password is the token generated by gerrit on your user settings page
+machine review.rdoproject.org login jonhdoe password changeme
+```
+
 ## Contributing
 
 Are you missing a feature, just check if there is a bug open for it and add