docs(contributing): How create or change SECRET_KEY

[pankajsoni22]

Explained how to change/create secret key.

SUMMARY

When you are running an instance of superset naturally you might have added many DataBase connections, created a lot of charts and dashboards.
Suddenly if you want to change the SECRET_KEY to make your data encrypted , you need current secret key that has already encrypted your data.
Then you need to create a new SECRET_KEY

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

BEFORE SS

AFTER SCREENSHOT

ADDITIONAL INFORMATION

To change the SECRET_KEY we need to provide current secret key as PREVIOUS_SECRET_KEY in superset_config.py and New secret key should be assigned to SECRET_KEY variable.