Section on public/private dependencies (#4)

* Start section on public/private dependencies

* Fix typo

* Re-explain the public/private extension (not finished)

* Tweaks

* Add explanation of dependency provider implementation

* Tweaks of the code example

* Change img path for better visibility in github

* Typo

* Adjust explanation of seed markers

* Improve explanation of PkgSeeds::Markers

* Add a remark about Markers.pkgs set

* Comment adjustments in get_dependencies example

* Add additional remarks at the end

* Actually remove the additional section

src/SUMMARY.md

@@ -11,6 +11,7 @@
 - [Advanced usage and limitations](./limitations/intro.md)
   - [Optional dependencies](./limitations/optional_deps.md)
   - [Allowing multiple versions of a package](./limitations/multiple_versions.md)
+  - [Public and Private packages](./limitations/public_private.md)
   - [Versions in a continuous space](./limitations/continuous_versions.md)
   - [Pre-release versions](./limitations/prerelease_versions.md)
 - [Internals of the PubGrub algorithm](./internals/intro.md)