The link was missing a `tutorials` in the path.

Only one of the two plugin system links seems to work, the other 404's. I think the proposed link may go to the equivalent place?

Current URL goes to 404 page, corrected URL to avoid confusion.

Addresses issue #1330

The provider is used for debugging to help displaying current rebar's state.

Usage:
  rebar3 state

- The README is better targetted to newcomers and explains the basics of
  the project: what it is, why use it, how to get started, where to find
  more information. It looks less like a reference and more like a brief
  overview of rebar3.
- Whatever looks like a list or reference manual material is expected to
  be moved to the website
- Be clearer about the rebar3 project structure in CONTRIBUTING.md;
  explain how to write tests, how to structure code, and so on.
- Added contributing section on bug reports, feature requests, etc.

Changelog since alpha-4:

- rebar3 now bootstraps itself
- app compile fixes on app info contents and config inheritance
- dropping ErlyDTL and going to Mustache for smaller executable sizes
  and faster load time on small CPUs
- dropping `erlydtl compile` command, ErlyDTL task moved to a plugin
- default dependencies are always fetched in the right place and don't
  need to be copied across profile runs
- escriptize fixes
- `unlock` command added
- resolving/avoiding clashes when a rebar3 dep and an application
  dependency conflict together
- profile deduplication (`rebar3 as test,test,prod,test eunit` returns
  the profile/directories for `prod+test`)
- `upgrade` command bugfixes to recognize promotion of transient deps to
  top-level deps
- support template when rebar3 is run as an embedded library
- some optimizations
- parse transform support is better in erl_opts
- fixes to package handling
- fixes to `rebar3 deps`