README.md: update instruction to build repo locally

[Slamdunk]

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[codecov]

Codecov Report

Merging #541 (b26f66b) into master (1eee4d7) will increase coverage by 0.18%.
The diff coverage is 78.57%.

@@             Coverage Diff              @@
##             master     #541      +/-   ##
============================================
+ Coverage     95.33%   95.52%   +0.18%     
+ Complexity      356      354       -2     
============================================
  Files            45       45              
  Lines           857      826      -31     
============================================
- Hits            817      789      -28     
+ Misses           40       37       -3     

Impacted Files	Coverage Δ
src/Message.php	88.00% <0.00%> (+1.08%)	⬆️
src/Message/Transcoder.php	86.66% <0.00%> (-1.91%)	⬇️
src/Connection.php	88.57% <50.00%> (+1.07%)	⬆️
src/Mailbox.php	96.77% <100.00%> (-0.20%)	⬇️
src/Message/AbstractPart.php	95.80% <100.00%> (-0.09%)	⬇️
src/Server.php	95.12% <100.00%> (-0.44%)	⬇️
src/Exception/AbstractException.php	92.30% <0.00%> (-1.03%)	⬇️
... and 5 more

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