Add root license

[st0012]

GitHub can't capture the repo's license from a link file. So I think we need to give it a copy of the actual license.

[codecov-commenter]

Codecov Report

Merging #1555 (268af63) into master (4e7eb21) will increase coverage by 0.37%.
The diff coverage is n/a.

❗ Current head 268af63 differs from pull request most recent head 19e7e4f. Consider uploading reports for the commit 19e7e4f to get more accurate results

@@            Coverage Diff             @@
##           master    #1555      +/-   ##
==========================================
+ Coverage   97.92%   98.29%   +0.37%     
==========================================
  Files         222      127      -95     
  Lines       10867     7030    -3837     
==========================================
- Hits        10641     6910    -3731     
+ Misses        226      120     -106     

Impacted Files	Coverage Δ
sentry-raven/lib/raven/transports/stdout.rb
sentry-raven/lib/raven/base.rb
sentry-raven/lib/raven/cli.rb
sentry-raven/lib/raven/processor/cookies.rb
sentry-raven/lib/raven/integrations/sidekiq.rb
sentry-raven/spec/raven/raven_spec.rb
sentry-raven/lib/raven/utils/request_id.rb
...aven/integrations/rails/controller_methods_spec.rb
...y-raven/spec/raven/processors/http_headers_spec.rb
sentry-raven/lib/raven/interface.rb
... and 85 more

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[rhcarvalho]

Thanks @st0012 !

Most other SDKs call this file LICENSE (without extension), if it makes no difference otherwise I'd rename the file to match the other SDKs.

[st0012]

@rhcarvalho 👍 updated. merging now.