rearranged documentation, added basic troubleshooting guide

docs/troubleshooting.md

@@ -0,0 +1,28 @@
+# Troubleshooting
+
+Generally if the following information doesn't provide a solution to your problem please take a look at existing issues (search for keywords) before creating a new one.
+
+## "No registration exists matching provided key"
+
+You probably changed from staging-CA to production-CA (or the other way).
+
+Currently letsencrypt.sh doesn't detect a missing registration on the selected CA,
+the current workaround is to move `private_key.pem` (and, if you care, `private_key.json`) out of the way so the scripts generates and registers a new one.
+
+This will hopefully be fixed in the future.
+
+## "Error creating new cert :: Too many certificates already issued for: [...]"
+
+This is not an issue with letsencrypt.sh but an API limit with letsencrypt.
+
+At the time of writing this you can only create 5 certificates per domain in a sliding window of 7 days.
+
+## Invalid challenges
+
+There are a few factors that could result in invalid challenges.
+
+If you are using http validation make sure that the path you have configured with WELLKNOWN is readable under your domain.
+
+To test this create a file (e.g. `test.txt`) in that directory and try opening it with your browser: `http://example.org/.well-known/acme-challenge/test.txt`.
+
+If you get any error you'll have to fix your webserver configuration.