add "How to" and troubleshooting to readme

Author: PhilWun

README.md

@@ -1 +1,30 @@
 # Pattern Atlas API
+
+## How to generate a new Liquibase changelog for changed database schema
+
+1. have a database instance running with the new schema on port 5060
+2. hava a database instance running with the old schema on port 5061
+
+create the file `liquibase.properties` with the following content:
+```
+url=jdbc:postgresql://localhost:5061/patternatlas
+username=patternatlas
+password=patternatlas
+referenceUrl=jdbc:postgresql://localhost:5060/patternatlas
+referenceUsername=patternatlas
+referencePassword=patternatlas
+liquibase.hub.mode=off
+```
+
+Run: `liquibase diff-changelog --changelog-file=new_changelog.xml`
+
+## Troubleshooting
+
+### Liquibase ChangelogParseException the file was not found
+
+Remove the `file:` prefix from the referenced files in the `patternatlas_full.xml` file.
+
+### Liquibase can't find `patternatlas_schema.xml`
+
+Copy all the Liquibase changelog files into a folder inside the project folder and include this folder in the classpath.
+This can be done for example in the run configuration in IntelliJ.