Readme: "infinite" value for _attempts

[swashbuck]

Subject of the issue

The readme indicates that "infinite" is a valid value for _attempts. However, the schemas require a numeric value. Therefore, the string "infinite" is not permitted in the authoring tool.

_attempts (number): Controls the number of attempts available to the user. Any of the following values may be used to indicate an infinite number of attempts: -1, 0, null, undefined, "infinite". Note: If set to "infinite" and used in conjunction with _completionCriteria._requireAssessmentCompleted = true the course will not be considered complete until the assessment is passed.

Although "infinite" may be valid when working in the framework, I feel that we should encourage developers to use a numeric value for compatibility with the authoring tool. Suggest updating the readme to remove references to the string "infinite" or clarifying when it can and cannot be used.