Clarify license model

[reyang]

Problem statement

Most parts of this project are under Apache-2.0 license, however, there are few exceptions:

• [SDK] Remove the dependency on M.E.Options 5.0 #4093 introduced DelegatingOptionsFactory.cs which is under MIT license.
• [SDK] Remove the dependency on M.E.C.EnvironmentVariables #4092 introduced three files under MIT license to handle environment variables.

Here is the query https://github.com/search?q=repo%3ACodeBlanch%2Fopentelemetry-dotnet%20%22under%20the%20MIT%20license%22&type=code.

The asks

1. Follow up and handle MIT license properly (e.g. confirm if we need to include MIT license in README/NuGet/etc.).
2. Check if there are any other license concerns/issues.
3. Update the CONTRIBUTING.md doc and perhaps ISSUE_TEMPLATE to clarify what should be the process for future PRs.

[reyang]

@CodeBlanch are there remaining issues?

[CodeBlanch]

@reyang I think we're good. I'll close this.