[Docs] Update README (#4)

Author: insanecoding

README.md

@@ -1,7 +1,26 @@
-# typescript-coverage
+# `Typescript Coverage`
+
+## Intro
+
+> A dead-simple criteria to track your Typescript migration.
+
+To measure Typescript coverage, we simply divide the total number of lines in TS by the total number of lines in the project.
+That's it, nothing more. Easy, huh?
+
+## Example output
+
+![An output example when run in terminal](/img/example.png)
+
+## Our approach
+
+- We locate all the files according to the `include` and `exclude` options.
+- If the file extension belongs to `tsExtensions`, we consider the whole file written in TS. Its line number is added to "lines in TS".
+- In any case, its line number is added to "all lines".
+- We divide "lines in TS" by "all lines" to get the final percentage.
 
 ## Installation
-Install it alongside its peer dependencies (`chalk` and `glob`)
+
+Install the package alongside its peer dependencies (`chalk` and `glob`) with your favorite package manager:
 
 ```bash
 npm install --save-dev typescript-coverage chalk glob
@@ -15,4 +34,49 @@ yarn add --dev typescript-coverage chalk glob
 pnpm install --save-dev typescript-coverage chalk glob
 ```
 
-⚠️You will need node 20 and pnpm for local development
+## Using in your project
+
+1. Add a new script to your `package.json`:
+
+```json
+{
+  "scripts": {
+    "typescript-coverage": "typescript-coverage"
+  }
+}
+```
+
+2. Create a `type-coverage.config.json` file in the root of your project to customize the coverage calculation (optional).
+   When no config was found, the following defaults will be used:
+
+```json5
+{
+  // how to find the files to analyze (glob syntax)
+  include: 'src/**/*.{ts,tsx,js,jsx}',
+  // the files or the folders to ignore: mocks, tests, etc. (glob syntax)
+  exclude: ['**/__tests__/**'],
+  // whether to ignore empty lines
+  ignoreEmptyLines: false,
+  // the files that are consider written in TS (could be .ts, .tsx, .mts, .cts, etc)
+  tsExtensions: ['.ts', '.tsx'],
+}
+```
+
+## System requirements
+
+- The project uses top-level `await` with ES modules and thus requires Node v14.8.0 or higher (tested on Node 18 and 20).
+- To run the tests locally, the PNPM package manager must be installed. Note: this requirement only applies if you are contributing to the project.
+
+## Caveats
+
+- For simplicity, we are only looking at the file extension but the content isn't validated. A linter or a Typescript compiler will take care of that.
+- If the file has a `.ts` extension but the content is not a valid TS code, it will still be treated as TS.
+- No type inference is taken into account.
+- If the file is a mixture of JS and TS code but has a `.ts` extension, all the lines will be counted as TS.
+- We don't ignore comments. Taking different comments into account (inline, multiline) would further complicate it.
+
+## Our story
+
+At Prose, we've been migrating two projects to Typescript.
+We needed the simplest way to track our progress, so we created `typescript-coverage` — a minimalistic tool focused on this sole goal.
+Instead of copying and pasting it into every project, we decided to make it an open-source package for anyone facing the same challenge 💚