Adding DESIGN.md (#135)

DESIGN.md

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+# HTTP Provider Design
+
+The HTTP Provider offers functionality for interacting with generic HTTP servers as part of terraform configuration.
+Specifically, the provider issues an HTTP GET request to the defined URL on every Terraform run.
+
+Below we have a collection of _Goals_ and _Patterns_: they represent the guiding principles applied during the
+development of this provider. Some are in place, others are ongoing processes, others are still just inspirational.
+
+## Goals
+
+* [_Stability over features_](.github/CONTRIBUTING.md)
+* Provide managed data source to issue HTTP GET request.
+* Support usage of requests to URLs directed to either `http` or `https`.
+* Support the supplying of request headers.
+* Expose response headers returned from request.
+* Expose response body as string where applicable.
+
+## Patterns
+
+Specific to this provider:
+
+* Only idempotent GET requests are supported.
+* Only 200 status codes are considered successful.
+
+General to development:
+
+* **Avoid repetition**: the entities managed can sometimes require similar pieces of logic and/or schema to be realised.
+ When this happens it's important to keep the code shared in communal sections, so to avoid having to modify code in
+ multiple places when they start changing.
+* **Test expectations as well as bugs**: While it's typical to write tests to exercise a new functionality, it's key to
+ also provide tests for issues that get identified and fixed, so to prove resolution as well as avoid regression.
+* **Automate boring tasks**: Processes that are manual, repetitive and can be automated, should be. In addition to be a
+ time-saving practice, this ensures consistency and reduces human error (ex. static code analysis).
+* **Semantic versioning**: Adhering to HashiCorp's own
+ [Versioning Specification](https://www.terraform.io/plugin/sdkv2/best-practices/versioning#versioning-specification)
+ ensures we provide a consistent practitioner experience, and a clear process to deprecation and decommission.