Initital pass at deps README (#4686)

* Initital pass at README

* Finished the sentence

* fixed typo and added changelog

CHANGELOG.md

@@ -18,6 +18,7 @@
 - Use mashumaro for serialization in event logging ([#4504](https://github.com/dbt-labs/dbt-core/issues/4504), [#4505](https://github.com/dbt-labs/dbt-core/pull/4505))
 - Drop support for Python 3.7.0 + 3.7.1 ([#4584](https://github.com/dbt-labs/dbt-core/issues/4584), [#4585](https://github.com/dbt-labs/dbt-core/pull/4585), [#4643](https://github.com/dbt-labs/dbt-core/pull/4643))
 - Re-format codebase (except tests) using pre-commit hooks ([#3195](https://github.com/dbt-labs/dbt-core/issues/3195), [#4697](https://github.com/dbt-labs/dbt-core/pull/4697))
+- Add deps module README ([#4686](https://github.com/dbt-labs/dbt-core/pull/4686/))
 
 Contributors:
 - [@NiallRees](https://github.com/NiallRees) ([#4447](https://github.com/dbt-labs/dbt-core/pull/4447))

core/dbt/deps/README.md

@@ -1 +1,35 @@
 # Deps README
+
+The deps module is responsible for installing dbt packages into dbt projects.  A dbt package is a standalone dbt project with models and macros that solve a specific problem area.  More specific information on dbt packages is available on the [docs site](https://docs.getdbt.com/docs/building-a-dbt-project/package-management).
+
+
+# What's a package?
+
+See [How do I specify a package?](https://docs.getdbt.com/docs/building-a-dbt-project/package-management#how-do-i-specify-a-package) on the docs site for a detailed explination of the different types of packages supported and expected formats.
+
+
+# Files
+
+## `base.py`
+
+Defines the base classes of `PinnedPackage` and `UnpinnedPackage`.
+
+`downloads_directory` sets the directory packages will be downloaded to.
+
+## `git.py`
+
+Extends `PinnedPackage` and `UnpinnedPackage` specific to dbt packages defined with git urls.
+
+## `local.py`
+
+Extends `PinnedPackage` and `UnpinnedPackage` specific to dbt packages defined locally.
+
+## `registry.py`
+
+Extends `PinnedPackage` and `UnpinnedPackage` specific to dbt packages defined on the dbt Hub registry.
+
+`install` has retry logic if the download or untarring process hit exceptions (see `dbt.utils._connection_exception_retry`).
+
+## `resolver.py`
+
+Resolves the package definition into package objects to download.