Update CONTRIBUTING.md after using git submodule and make

[songy23]

Follow up of #504 (comment).

[codecov-io]

Codecov Report

Merging #507 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #507     +/-   ##
===========================================
+ Coverage     78.34%   78.55%   +0.2%     
- Complexity      560      561      +1     
===========================================
  Files            65       65             
  Lines          1926     1926             
  Branches        188      188             
===========================================
+ Hits           1509     1513      +4     
+ Misses          357      355      -2     
+ Partials         60       58      -2

Impacted Files	Coverage Δ	Complexity Δ
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	100% <0%> (+1.2%)	56% <0%> (+1%)	⬆️
...y/sdk/trace/export/BatchSampledSpansProcessor.java	92.04% <0%> (+2.27%)	6% <0%> (ø)	⬇️

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