Adds nhs-numbers Python package to README.md (#81)

I worked on this project last summer but forgot to submit it until now!

README.md

@@ -149,6 +149,7 @@ Curated list of awesome open source healthcare software, libraries, tools and re
   * [Health data standards](https://github.com/projectcypress/health-data-standards) - Ruby library for generating and consuming various healthcare related formats. These include HITSP C32, QRDA Category I, and QRDA Category III.
   * [Hermes](https://github.com/wardle/hermes) - a SNOMED CT terminology server. 
   * [MITK](https://www.mitk.org/wiki/The_Medical_Imaging_Interaction_Toolkit_(MITK)) - The Medical Imaging Interaction Toolkit.
+  * [nhs-numbers](https://github.com/uk-fci/nhs-number) - Python package to provide utilities for NHS Numbers, including validity checks, normalisation, and generation.
   * [Node HL7](https://github.com/MatthewVita/node-hl7-complete) - Node module that is bridged with the Java Hapi HL7 library.
   * [Node-hl7-parser](https://github.com/RedoxEngine/redox-hl7-v2) - Open source version of Redox's HL7 v2 to schema-fied JSON parser.
   * [php-fhir](https://github.com/dcarbone/php-fhir) - Tools for creating PHP classes from the HL7 FHIR Specification.