Clarify documentation about which options are mandatory

Hi

The documentation on https://lwc-gus-bot.herokuapp.com currently says:

> In order to customize your repo for your team you have to commit a configuration file .git2gus/config.json with specific data about your team. e.g.
>
> {
> "productTag": "a1aB0000000Jgm7IAC",
> "defaultBuild": "218"
> }

And then later:

> ## Configuring Git2Gus
> ...
> If a default productTag isn't provided in your config (ex: if the line "productTag": "a1aB0000000Jgm7IAC" wasn't there) and no matching productTagLabel is found, the work item will not be synced. In the example below, the GitHub label "area:lwc" corresponds to the productTag "1234".
> ...
> `defaultBuild` The build number the application is going to use by default when it creates a new work item. Notice this value might need to be changed every release.

After reading this it still wasn't 100% clear whether `defaultBuild` was required or not. I suspected it was, but had to check the source to be sure:
https://github.com/forcedotcom/git2gus/blob/ab66c318378ac2e7561bd6c39baca5f515b1b4c6/api/services/Github/getConfig.js#L17-L28

I think the following changes might make this clearer:
1. Replace `In order to customize your repo for your team` with something like `In order for Git2Gus to be enabled`
2. In the "Configuring Git2Gus" section, add something like `this option must be specified` to the `defaultBuild` definition.

Many thanks :-)