Update usage instructions and separate docs from README

doc/migrations/0.2.0.md

@@ -0,0 +1,43 @@
+# Migration from 0.1.x to 0.2.x
+
+If you used this client before `0.2.0`, you have a different directory structure than the current one. If you want to upgrade, but keep all your data, here's a migration guide.
+
+```bash
+# Start in ./data
+cd data
+
+# Move your account key to new location:
+
+mkdir accounts
+mv account/key.pem accounts/acme-v01.api.letsencrypt.org.directory.pem
+# or accounts/acme-staging.api.letsencrypt.org.directory.pem if it's a staging key
+
+# account should now be empty or contain just a config.json, you can delete the folder then
+rm -rf account
+
+# Migrate certificates to new location:
+
+cd certs
+mkdir acme-v01.api.letsencrypt.org.directory
+
+# Move all your certificate directories
+# Repeat for all directories!
+mv example.com acme-v01.api.letsencrypt.org.directory
+# or acme-staging.api.letsencrypt.org.directory
+
+# Delete all config.json files which may exist
+find -name "config.json" | xargs rm
+
+# Update to current version
+# Alternatively have a look at the new installation instructions and use the Phar
+git checkout master && git pull
+
+# Check out latest release
+git checkout $(git describe --tags `git rev-list --tags --max-count=1`)
+
+# Update dependencies
+composer update --no-dev
+
+# Reconfigure your webserver to use the new paths
+# and check (and fix) your automation commands.
+```