Document the setup mistakes behind "Could not find table 'solid_errors'"

[fractaledmind]

Closes #73.

The reports in #73 come from three setup mistakes, none of them a bug in the gem. This adds a README note for each:

• db:create + db:migrate empties db/errors_schema.rb. Solid Errors ships a schema, not migrations. So db:migrate leaves the errors database empty and then dumps that empty schema over the file (two commenters hit this). The install section now says to use db:prepare, and how to recover.
• The gem is loaded in an environment with no errors database. It writes to the primary database and fails. config.solid_errors.enabled = false (from Only subscribe to Rails.error if enabled #96, released in 0.8.0) is now recommended for those environments.
• Configuration in config/development.rb. Rails never loads that file, which is why SolidErrors.connects_to was nil in the original report.

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