Update install step in README

[artur-intech]

Mention requiring the gem. This may be especially helpful for the users
of the old sentry-raven gem as someone can accidently forget updating
require statement as a part of a migration.

[solnic]

Thanks. I'm pretty sure this was originally skipped because in Rails things get auto-required via Bundler.require but it's not always the case, so this can be indeed helpful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.01%. Comparing base (90d5a3a) to head (2e9623d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2638      +/-   ##
==========================================
- Coverage   96.13%   95.01%   -1.13%     
==========================================
  Files         133      133              
  Lines        5123     5092      -31     
==========================================
- Hits         4925     4838      -87     
- Misses        198      254      +56     

Components	Coverage Δ
sentry-ruby	94.94% <ø> (-2.45%)	⬇️
sentry-rails	94.99% <ø> (+0.08%)	⬆️
sentry-sidekiq	94.25% <ø> (+14.17%)	⬆️
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	98.62% <ø> (ø)

see 46 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.