update readme with info about reading vcf 4.3

[lbergelson]

Description

Updating the readme to mention that we can read 4.3.

Checklist

• [ ] Code compiles correctly
• [ ] New tests covering changes and new functionality
• [ ] All tests passing
• [ ] Extended the README / documentation, if necessary
• [ ] Is not backward compatible (breaks binary or source compatibility)

[codecov-io]

Codecov Report

Merging #1381 into master will increase coverage by 0.009%. The diff coverage is n/a.

@@               Coverage Diff               @@
##              master     #1381       +/-   ##
===============================================
+ Coverage     67.964%   67.973%   +0.009%     
- Complexity      8346      8347        +1     
===============================================
  Files            570       570               
  Lines          33846     33846               
  Branches        5667      5667               
===============================================
+ Hits           23003     23006        +3     
+ Misses          8657      8655        -2     
+ Partials        2186      2185        -1

Impacted Files	Coverage Δ	Complexity Δ
...samtools/util/AsyncBlockCompressedInputStream.java	76% <0%> (+4%)	13% <0%> (+1%)	:arrow_up: