Update README.md

[duncup]

The pull request will be closed without any reasons if it does not satisfy any of following requirements:

1. Make sure you are targeting the master branch, pull requests on release branches are only allowed for bug fixes.
2. Please read contributing guidelines: CONTRIBUTING
3. Describe what your pull request does and which issue you're targeting (if any and Please use English)
4. ... if it is not related to any particular issues, explain why we should not reject your pull request.
5. The Commits must use English, must be test and No useless submissions.

You MUST delete the content above including this line before posting, otherwise your pull request will be invalid.

Please provide Issues links to:

Provide test code:

Description

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[vcaesar]

In fact, if node installed go and gcc, you only need to execute npm install robotn. You do not need to download other tools and build from source code.

[vcaesar]

Thanks for your PR, I updated README.md, deleted the unclear statement; Because package.json performs these operations, I am considering whether to add go get -v github.com/go-vgo/robotgo.