add how to install yarn via apt for Ubuntu users.

[fujitatomoya]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Which issue(s) this PR fixes:

Fixes #571

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Quick guide for ubuntu user how to install yarn.

• What is the current behavior? (You can also link to an open issue here)

Not mentioned, not friendly for ubuntu user to get started.

• What is the new behavior (if this is a feature change)?

N.A

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

N.A

[fujitatomoya]

my assumption here is that most likely developers would use ubuntu platform, this could be more developer friendly procedure to set up the doc dev environment.

[Shelley-BaoYue]

/lgtm
Looks great! And I will update to the Chinese version according to this PR. Is it okay?

[fujitatomoya]

@Shelley-BaoYue

Looks great! And I will update to the Chinese version according to this PR. Is it okay?

yes, please.

before you do that, i will try to address @wbc6080 's comments.

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[fujitatomoya]

@wbc6080 i use ubuntu, and confirmed both procedure works without any problems.

[wbc6080]

@wbc6080 i use ubuntu, and confirmed both procedure works without any problems.

OK, thanks for your contribution!
/lgtm