feat(docs): Added docs

README.md

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+# Email Newsletter API
+
+- An API to enable blog visitors subcribe to the newsletter so that they can receive updates when new content is published on the blog.
+
+## Development setup
+
+- Install [cargo-watch](https://crates.io/crates/cargo-watch).
+- Open your favorite text editor (better be neovim or emacs).
+- Run `cargo watch -x check -x test -x run` to lint, test and run the binary as soon as you make a change to the file.
+- Bonus: install and use `mold`, a very fast linker that can link your Rust binary _blazingly fast_.
+
+## [Technical Write Up](./docs/technical_write_up.md)

docs/technical_write_up.md

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+# Technical Write Up
+
+## Routes
+
+- `health_check`: returns a HTTP code 200 if the server is up and running. Response body is empty.
+
+## The `tokio` Async Runtime
+
+`tokio` will be the engine in charge of driving Futures to completion
+
+- `#[tokio::main]` macro is just a builder to help building the runtime. It is basically saying, we want this async function to be run.
+- `tokio::spawn(/*async task*/)` will spawn an async task to be run. We can continue executing other code concurrently while `task` runs in the background. If we need to wait for `task` to complete before proceeding, we can use `task.await` (which `#[tokio::test]` will take care for us in the mean time).
+
+### Testing with `tokio`
+
+- Each test has to be marked `#[tokio::test]`.
+- This macro will build a new async runtime for that one test.
+- By calling `spawn_app()`, we will get a API HttpServer running for the entire duration of the single test that the macro above belongs to.
+
+## The Test Suite
+
+- The OS will find an available port for the test suite to use.