Update contribution instructions.

[SomeoneToIgnore]

This change is 

[gilberto-torrezan]

Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, 3 unresolved discussions, some commit checks failed.

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CONTRIBUTING.md, line 118 at r1 (raw file):

This also goes for features that are hard to test automatically.

After submitting a pull request, our CI system will trigger the verification build automatically, including integration tests and you will be able to see the whole build output and results.

Is it possible to provide a link to our CI system here?

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README.md, line 75 at r1 (raw file):

To run IT tests locally, you'll need a [Testbench](https://vaadin.com/testbench) license and a Chrome browser installed (currently this is the only browser that IT tests are run in).
If you don't have the license, it's ok, our CI will run those tests for you after you create a pull request. 

... our CI servers will run. CI alone is a bit strange

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README.md, line 79 at r1 (raw file):

When running IT tests locally, by default, local Chrome is used to run tests.
The other opportunity is to run tests using test hub, to do so, you need to start a hub and start tests with `-Dtest.use.hub=true -Dcom.vaadin.testbench.Parameters.hubHostname="foo"` properties.

Do we have a link about these test hubs and how do they work (and what is the foo property there)? If so, you could put it here.

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Comments from Reviewable

[SomeoneToIgnore]

Review status: all files reviewed at latest revision, 3 unresolved discussions.

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CONTRIBUTING.md, line 118 at r1 (raw file):

Previously, gilberto-torrezan (Gilberto Torrezan) wrote…

Is it possible to provide a link to our CI system here?

I would prefer to avoid it:

1. Current TeamCity login page has rather small button for guest login, should be explained also, rather bothersome
2. If we migrate to another CI, we would be forced to change it everywhere
3. What value does it bring to the users? They will get the url when submin the PR.

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README.md, line 75 at r1 (raw file):

Previously, gilberto-torrezan (Gilberto Torrezan) wrote…

... our CI servers will run. CI alone is a bit strange

Well, strictly speaking, CI servers are not ours. Have adjusted it a bit.

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README.md, line 79 at r1 (raw file):

Previously, gilberto-torrezan (Gilberto Torrezan) wrote…

Do we have a link about these test hubs and how do they work (and what is the foo property there)? If so, you could put it here.

Removed completely, doubt that an external contributor would pick launching Selenium cluster over installing Chrome.

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Comments from Reviewable

[gilberto-torrezan]

Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable