Add missing documentation

[BYK]

We need to add the following documentation pieces:

• [ ] The http://spotlight@localhost:8969/0 DSN hack for unsupported SDKs
• [ ] The LAUNCH_EDITOR and other env variables for "Open in editor" feature
• [ ] Make it clear that you want Sentry SDKs for the best value (related #243)

[BYK]

Related #543