Add Premise to readme.md

[jmccallum-premise]

Adds premise to readme.md only

[codecov-io]

Codecov Report

Merging #5568 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #5568     +/-   ##
========================================
+ Coverage   78.89%     79%   +0.1%     
========================================
  Files         489     489             
  Lines       30741   30741             
========================================
+ Hits        24254   24286     +32     
+ Misses       6487    6455     -32

Impacted Files	Coverage Δ
airflow/models/dagbag.py	92.12% <0%> (+0.46%)	⬆️
airflow/utils/dag_processing.py	58.25% <0%> (+1.06%)	⬆️
airflow/jobs/scheduler_job.py	70.38% <0%> (+1.24%)	⬆️
airflow/executors/__init__.py	66.66% <0%> (+4.16%)	⬆️
airflow/utils/sqlalchemy.py	79.06% <0%> (+4.65%)	⬆️
airflow/executors/sequential_executor.py	100% <0%> (+52.38%)	⬆️

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[ashb]

Welcome aboard! 👋