Insufficient setup guide

When trying to use the `git-http-mock-server` I am unable to start the server.
First I initialized my directory with `npm init` and provided it with some pseudo information.
```json
{
"name": "example",
"version": "1.0.0",
"description": "",
"main": "index.js",
"dependencies": {},
"devDependencies": {
"git-http-mock-server": "^2.0.0"
},
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1"
},
"author": "",
"license": "ISC"
}
```

Then followed the README and installed the `git-http-mock-server` with:
```bash
npm install --save-dev git-http-mock-server
```

When executing `git-http-mock-server` it returns:
```bash
command not found: git-http-mock-server
```

Running it with `npx` fixes the problem but this is not documented in the README.
```
npx git-http-mock-server
```

Start with the README instructions for installing and starting git-http-mock-server. Document that a locally installed command can be run with npx git-http-mock-server, then verify the documented command matches the reported working invocation.

Written by the indexing model from the issue text.