Make the .all-contributorsrc file optional (infer contributors from README.md)

**Is your feature request related to a problem? Please describe.**

Right now, only some of the [CLI and bot configuration in `.all-contributorsrc`](https://allcontributors.org/docs/en/bot/configuration) values can be inferred automatically. The `contributors` array in particular is always stored in the `.all-contributorsrc` file, which is always created for users.

**Describe the solution you'd like**

I think most repos could actually skip the `.all-contributorsrc` file altogether and infer much of the same data from the `README.md`. Since the table has all the same data as `contributors` -user photos, contribution types, etc.- the data is essentially being stored twice.

Proposal: let's make `.all-contributorsrc` optional, and not created by commands it it doesn't yet exist?

**Describe alternatives you've considered**

Hmm... configurable presets, distributed as npm packages? 🤷

**Additional context**

I think this ties into a general trend of "config file fatigue". A lot of users find it exhausting and overwhelming to open a new repository with many config files. I think anything we can do to reduce the vertical height of a github.com repository root files display is helpful. ❤️