Move link to docs under it's own heading

[ghost]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: Update to documentation.

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

This PR moves the link to the documentation site under it's own heading in the repo's README.

The reason I submitted this PR is that I often have trouble finding the Vue docs.
When googling I click the Vue github repo link by default.
When landing on the github repo I am confused as to where the docs are.
Currently there is a link to the documentation site, however it is not very noticeable.
In order to make it more noticeable and easier for other developers to find, I have moved the link to the documentation under it's own heading.

This will make it easier for developers to quickly find a link to the Vue documentation from the github repo.