Just cleaning up the README a little

[markiantorno]

Added proper links to azure pipelines for main and snapshot as well as some cleanup.

[codecov]

Codecov Report

Merging #2322 (9e5d079) into master (66e6cab) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2322      +/-   ##
============================================
- Coverage     80.98%   80.98%   -0.01%     
  Complexity    17329    17329              
============================================
  Files          1198     1198              
  Lines         63959    63959              
  Branches       9900     9900              
============================================
- Hits          51797    51795       -2     
- Misses         8220     8221       +1     
- Partials       3942     3943       +1     

Impacted Files	Coverage Δ	Complexity Δ
...e/src/main/java/ca/uhn/fhir/parser/JsonParser.java	81.33% <0.00%> (-0.25%)	212.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 66e6cab...9e5d079. Read the comment docs.