Include the HL7 disclaimer in the README.md file

[jingtang10]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Description
Include the HL7 disclaimer in the README.md file as recommended here: http:https://www.hl7.org/legal/trademarks.cfm?ref=nav

Alternative(s) considered
NA

Type
Documentation

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct
• I have read the Contributing page
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally
• I have built and run the reference app(s) to verify my change fixes the issue and/or does not break the reference app(s)

[codecov]

Codecov Report

Merging #1174 (77bd2a1) into master (0f497fa) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1174   +/-   ##
=========================================
  Coverage     84.54%   84.54%           
  Complexity      614      614           
=========================================
  Files           135      135           
  Lines         10299    10299           
  Branches        763      763           
=========================================
  Hits           8707     8707           
  Misses         1212     1212           
  Partials        380      380           

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