Add roadmap to README.md

[woop]

Moves the roadmap to our README.md and restructures it.

[codecov-commenter]

Codecov Report

Merging #1843 (2a68d86) into master (17d1874) will decrease coverage by 20.71%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1843       +/-   ##
===========================================
- Coverage   84.45%   63.73%   -20.72%     
===========================================
  Files          90       90               
  Lines        6772     6773        +1     
===========================================
- Hits         5719     4317     -1402     
- Misses       1053     2456     +1403     

Flag	Coverage Δ
integrationtests	?
unittests	63.73% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
.../integration/online_store/test_universal_online.py	18.03% <0.00%> (-81.97%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	18.18% <0.00%> (-81.82%)	⬇️
...fline_store/test_universal_historical_retrieval.py	20.00% <0.00%> (-80.00%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
sdk/python/feast/infra/online_stores/datastore.py	30.32% <0.00%> (-63.12%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	30.35% <0.00%> (-62.50%)	⬇️
sdk/python/tests/data/data_creator.py	38.09% <0.00%> (-61.91%)	⬇️
sdk/python/feast/infra/online_stores/dynamodb.py	30.37% <0.00%> (-60.76%)	⬇️
...n/feature_repos/universal/data_sources/bigquery.py	37.20% <0.00%> (-60.47%)	⬇️
... and 41 more

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[felixwang9817]

/lgtm

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