Update CONTRIBUTING.md

[yutongp]

No description provided.

[zjshen14]

Shall we not add details to the contributing.md?

See the examples bellow:

https://github.com/apache/spark/blob/master/CONTRIBUTING.md
https://github.com/prestodb/presto/blob/master/CONTRIBUTING.md

Instead, add it to https://github.com/iotexproject/iotex-core/wiki

[codecov]

Codecov Report

Merging #666 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
- Coverage   60.09%   60.07%   -0.03%     
==========================================
  Files         132      132              
  Lines       12560    12548      -12     
==========================================
- Hits         7548     7538      -10     
+ Misses       3972     3970       -2     
  Partials     1040     1040

Impacted Files	Coverage Δ
pkg/keypair/keypair.go	65.21% <0%> (-8.98%)	⬇️
consensus/scheme/rolldpos/rolldposctx.go	75.11% <0%> (-0.46%)	⬇️
pkg/keypair/secp256k1.go	90.9% <0%> (+7.57%)	⬆️

---

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[codecov]

Codecov Report

Merging #666 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
- Coverage   60.09%   60.06%   -0.04%     
==========================================
  Files         132      132              
  Lines       12561    12561              
==========================================
- Hits         7549     7545       -4     
- Misses       3972     3975       +3     
- Partials     1040     1041       +1

Impacted Files	Coverage Δ
db/trie/branchnode.go	65.59% <0%> (-1.61%)	⬇️
consensus/scheme/rolldpos/rolldposctx.go	75.11% <0%> (-0.46%)	⬇️

---

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 d458de5...19a90fe. Read the comment docs.