readme needs improvement

[hilliao]

1. The Scio broke in the Readme content
2. The Examples can't run easily. The following command after I changed CLASS_PATH dbeam-core_2.12.jar to dbeam/target/scala-2.12/*.jar, I got error Error: Unable to initialize main class com.spotify.dbeam.JdbcAvroJob
   Caused by: java.lang.NoClassDefFoundError: org/slf4j/Logger

java -cp CLASS_PATH dbeam-core_2.12.jar com.spotify.dbeam.JdbcAvroJob \
  --output=gs://my-testing-bucket-name/ \
  --username=my_database_username \
  --password=secret \
  --connectionUrl=jdbc:postgresql://some.database.uri.example.org:5432/my_database \
  --table=my_table

Can you provide more concrete examples? I am trying to figure out how to run from reading ./dbeam-pack/target/pack/bin/jdbc-avro-job. An example of how to run it in Google cloud dataflow with mvn compile exec:java would be nice.

3. Instead of running ./dbeam-pack/target/pack/bin/jdbc-avro-job, how about a full example?

bash -v ./dbeam-pack/target/pack/bin/jdbc-avro-job --connectionUrl=jdbc:postgresql://some.database.uri.example.org:5432/my_database --table=my_table --output=gs://my-testing-bucket-name/  --username=my_database_username --password=secret