Add documentation about inlinining sources inside source maps

[benvinegar]

Our documentation is not clear that both source maps and source files need to be accessible by Sentry, and users are sometimes confused when dealing with source map issues inside the app.

This does two things:

• adds --source-map-include-source to the UglifyJS2 example
• adds an admonition about why inline sources are preferred

Note that Webpack produces inline sources by default (can't seem to figure out if this is even configurable).

cc @mattrobenolt

[mattrobenolt]

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