Include in User Guide / sample scripts how to save to Cosmos DB

While the example to write to Cosmos DB is in CosmosDBDataFrameSpec.scala - this is not at all obvious. Should provide write example directly in readme.md and user guide.

To save a DataFrame to Cosmos DB based on the flights example used thorughout this documentation:

// Import SaveMode so you can Overwrite, Append, ErrorIfExists, Ignore
import org.apache.spark.sql.{Row, SaveMode, SparkSession}

// Create new DataFrame `df` which has slightly flights information 
// i.e. change the delay value to -999
val df = spark.sql("select -999 as delay, distance, origin, date, destination from c limit 5")

// Save to Cosmos DB (using Append in this case)
//    Ensure the baseConfig contains a Read-Write Key
//    The key provided in our examples is a Read-Only Key
df.write.mode(SaveMode.Append).cosmosDB(baseConfig)

[dennyglee]

Forgot to note here’s the Python version of it:

df.write.format('com.microsoft.azure.cosmosdb.spark').mode('overwrite').options(**writeConfig).save()

thanks @jangcy !

[dennyglee]

@ttelfer for **writeConfig, please refer to https://github.com/Azure/azure-cosmosdb-spark/wiki/Configuration-references. We’re in the process of updating our documentation to help find these things easier.