Add scripts documentation

[moniika]

Where
A scripts.md file at the top level of the blockly directory (same level as the package.json file).

What
Blockly is missing clear documentation for the script commands defined in package.json.

• Text
• Image or Gif
• Other

Old content
none

Suggested content
A markdown file, similar to https://github.com/google/blockly-samples/blob/master/scripts.md, that describes the available commands and, if useful, when they should be used.

Additional context
Issue was brought up in #4968

[rachel-fenichel]

Let's actually put this in the scripts directory, to avoid having more stuff at the top level.

[moniika]

Let's actually put this in the scripts directory, to avoid having more stuff at the top level.

There are a lot of things at the top level, but I think it's more intuitive to look for it there because the intention is to document the script tags defined in package.json scripts and not directly the gulpfile commands (eg: we would describe npm run build and not gulp build here). Also it would be more consistent with Blockly Samples that way.

If we do move it down, it might be worth considering at least linking scripts.md in README.md to make it easier to find.

[cpcallen]

Yes: I think referencing it in README.md is the way to go.

[cpcallen]

See also #5040.