Update README.md: add note to start clean after dependency updates.

[seltzered]

What it does

Proposing a note in the readme for those doing manual builds to run git clean -xfd when updating to a newer commit of the repo, particularly when updating dependencies. This is to help those running into the issue where they notice a runtime issue on electron build of "Error module did not self-register" ( drivelist.node ).

The jenkinsfile for what is (was?) the 'next' build has also done this according to #349 (comment) . As a casual developer (not using Jenkins) who comes back to working on Theia intermittently this issue has come back to frustrate me for a while and seems to have come up in other conversations (see eclipse-theia/theia#13126 ). There may be some better ways to handle this but the git clean -xfd approach has seemingly been more reliable and think there should be some note to help those getting started with theia-blueprint.

How to test

I suspect the issue others experience can be reproduced by:

• Try checking out theia-blueprint with an earlier commit (e.g. around 1.38.0) and manually building theia-blueprint
• Pull to a newer commit (e.g. 1.53.2) with dependency changes.
• Try manually building again, to making an electron build (preview or package), even with 'yarn clean' at the beginning.
• Try running the electron build, observe if you have drivelist.node runtime errors as others have experienced in Error module did not self-register while starting browser or electron app theia#13126 .

Review checklist

• [See NOTE] as an author, I have thoroughly tested my changes and carefully followed the review guidelines

NOTE: This is a readme change just suggesting to clean out data before build after changes. Open to feedback if there's a better way to approach this, but so far from my brief tests doing git clean -xfd seems to help reduce the chance of encountering the ""Error module did not self-register" runtime issue after making changes (either by updating dependencies by hand or by merging/pulling to newer commits).

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines