Make it easier to contribute to www with VSCode

[taras]

Related #893

Motivation

We want it to be easy for people to contribute to the website using VSCode.

Approach

Due to denoland/vscode_deno#787, imports are only read from the workspace's default deno.json file. Fortunately, we can trick VSCode Deno Plugin into reading the deno.json file from 'www' directory by creating a .code-workspace file that has www directory as the first in the list of folders. Since the core doesn't have dependencies, this issue will not effect the core folders.

1. created www/www.code-workspaces file with www as first and main folder
2. Added www/.vscode/settings.json with deno.enable: true to activate deno when it's root
3. Added www directory to excludes .vscode/settings.json
4. Added instructions to README.md that encourage users to read www/README.md for website contribution instructions
5. Added instructions on how to use the www/www.code-workspaces file for contributing to the website.

[cowboyd]