Update to overview. (#348)

* Update to overview.

* Update docs/overview.md

Co-authored-by: Stephen MacVicar <Jammjammjamm@users.noreply.github.com>

* Fix link wraps.

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Co-authored-by: Stephen MacVicar <Jammjammjamm@users.noreply.github.com>

docs/overview.md

@@ -13,7 +13,69 @@ nav_order: 2
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 ## Overview
 Inferno is framework for creating, executing, and sharing tests for health IT
-systems providing standardized FHIR APIs.
+systems providing standardized FHIR APIs.  You use Inferno to create
+**Inferno Test Kits** which are standalone, portable test applications that
+are tailored to test specific FHIR-enabled data exchange use cases as described
+in [FHIR Implementation Guides](http://fhir.org/guides/registry/).
+
+Examples of test applications (Test Kits) built using Inferno include:
+* The [US Core Test Kit](https://github.com/inferno-framework/us-core-test-kit)
+  provides tests for servers implementing [US Core Implementation
+  Guide](http://hl7.org/fhir/us/core/).
+* The [SMART App Launch Test Kit](https://github.com/inferno-framework/smart-app-launch-test-kit)
+  provides tests for systems that support the
+  [SMART App Launch Framework](http://hl7.org/fhir/smart-app-launch/index.html).
+* The [ONC Certification (g)(10) Standardized API Test Kit](https://github.com/onc-healthit/onc-certification-g10-test-kit)
+  provides tests for systems that seek conformance to
+  [ONC's Standardized API certification criterion](https://www.healthit.gov/test-method/standardized-api-patient-and-population-services).
+
+Each of these Test Kits is a standalone application that can be run on an end
+user's machine, on a shared private host behind a firewall, or on a shared
+public host such as [inferno.healthit.gov](https://inferno.healthit.gov/suites).
+They are Ruby applications built with the **Inferno Core** gem which provides
+several features useful for a FHIR testing application:
+
+* **Inferno DSL**: A domain specific language for authoring FHIR API tests that
+  includes a FHIR client, native Ruby classes for FHIR, and FHIR instance validators
+* **API, Web and CLI Interfaces**: Multiple interfaces for executing tests and
+  retrieving results
+* **Test Reuse**: Methods for reusing tests within a project or from other projects
+
+Inferno's design reflects the flexibility provided by the base FHIR
+specification.  As a platform specification that is intended to provide support
+across the whole healthcare process, FHIR relies on Implementation Guides to
+provide enough specificity to enable meaningful interoperability between actors
+for a given data exchange use case.  FHIR Implementation Guides typically will
+describe how to use relevant portions of the FHIR specification for
+its use case, while also providing additional rules that are not described
+within FHIR itself.  There are no limits to what these rules may include, and
+may even require the use of other standards, such as
+[OAuth](https://www.hl7.org/fhir/smart-app-launch/) or [QR Codes for Vaccination
+Credentials](https://build.fhir.org/ig/HL7/fhir-shc-vaccination-ig/), as part of
+their conformance criteria.
+
+Creating a test system that is flexible enough to test any arbitrary
+conformance criteria introduced in Implementation Guides is challenging.
+Inferno accomplishes this by providing test authors with a full featured
+Ruby programming environment to define and run their tests, and doesn't restrict
+usage of the rich ecosystem of open source third-party libraries provided by Ruby.
+This makes Inferno well-suited for testing Implementation Guides 
+that:
+
+* include the use of additional standards beyond FHIR,
+* have large specifications that could benefit from Ruby's meta-programming
+  capabilities to ease maintenance burden,
+* or require complex logic to thoroughly validate API responses.
+
+Inferno Core provides common utilities for FHIR-based testing, but tests are not
+limited to what is provided by Inferno Core.  Inferno Core's goal is to expand on
+the set of common utilities it provides for the benefit of the community.
+
+While Ruby is sufficient for most testing needs, Inferno Test Kits may also
+include components that are not defined in Ruby.  Inferno Test Kits can include
+other non-Ruby services as needed, and by default provides a Docker Compose
+configuration to run and integrate with these services from within Inferno's
+testing DSL.
 
 ## Documentation Resources
 - View the [JSON API Documentation](/inferno-core/api-docs) for information on