Add examples and further notes about duplicate policy

[filipecosta90]

Since RedisTimeSeries 1.4 we've added the ability to back-fill time series, with different duplicate policies.

However, we still see several issues being raised on the core repo/client repo that point to users no being aware of it. Example: RedisTimeSeries/redistimeseries-py#86

We should address this by adding examples and further notes about duplicate policy.
You can check our docs about duplicate policy here: https://oss.redislabs.com/redistimeseries/configuration/#duplicate_policy.

Sample Readme of python client with an example of the expected outcome of this documentation/examples task:
https://github.com/RedisTimeSeries/redistimeseries-py#further-notes-on-back-filling-time-series