docs(README): add slack

[L2ncE]

What type of PR is this?

docs

Check the PR title.

• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.

More detail description for this PR(en: English/zh: Chinese).

en: Add Slack to README.
zh: 在 README 中添加 Slack 。

[codecov]

Codecov Report

Base: 66.14% // Head: 66.09% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (fe0f605) compared to base (f180815).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #337      +/-   ##
===========================================
- Coverage    66.14%   66.09%   -0.05%     
===========================================
  Files           85       85              
  Lines         8317     8317              
===========================================
- Hits          5501     5497       -4     
- Misses        2456     2460       +4     
  Partials       360      360              

Impacted Files	Coverage Δ
pkg/common/timer/timer.go	61.90% <0.00%> (-19.05%)	⬇️

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[li-jin-gou]

Good job!