README: document all the source types

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@@ -2,6 +2,7 @@
 
 krops is a lightweigt toolkit to deploy NixOS systems, remotely or locally.
 
+
 ## Some Features
 
 - store your secrets in [password store](https://www.passwordstore.org/)
@@ -9,6 +10,7 @@ krops is a lightweigt toolkit to deploy NixOS systems, remotely or locally.
 - minimal overhead (it's basically just `nixos-rebuild switch`!)
 - run from custom nixpkgs branch/checkout/fork
 
+
 ## Minimal Example
 
 Create a file named `krops.nix` (name doesn't matter) with following content:
@@ -54,10 +56,91 @@ and run `$(nix-build --no-out-link krops.nix)` to deploy the target machine.
 Under the hood, this will make the sources available on the target machine
 below `/var/src`, and execute `nixos-rebuild switch -I /var/src`.
 
+
+## Source Types
+
+### `derivation`
+
+Nix expression to be built at the target machine.
+
+Supported attributes:
+
+* `text` -
+  Nix expression to be built.
+
+
+### `file`
+
+The file source type transfers local files (and folders) to the target
+using [`rsync`](https://rsync.samba.org/).
+
+Supported attributes:
+
+* `path` -
+  absolute path to files that should by transfered
+
+* `useChecksum` (optional) -
+  boolean that controls whether file contents should be checked to decide
+  whether a file has changed.  This is useful when `path` points at files
+  with mangled timestamps, e.g. the Nix store.
+
+
+### `git`
+
+Git sources that will be fetched on the target machine.
+
+Supported attributes:
+
+* `url` -
+  URL of the Git repository that should be fetched.
+
+* `ref` -
+  Branch / tag / commit that should be fetched.
+
+* `clean.exclude` -
+  List of patterns that should be excluded from Git cleaning.
+
+
+### `pass`
+
+The pass source type transfers contents from a local
+[password store](https://www.passwordstore.org/) to the target machine.
+
+Supported attributes:
+
+* `dir` -
+  absolute path to the password store.
+
+* `name` -
+  sub-directory in the password store.
+
+
+### `pipe`
+
+Executes a local command, capture its stdout, and send that as a file to the
+target machine.
+
+Supported attributes:
+
+* `command` -
+  The (shell) command to run.
+
+### `symlink`
+
+Symlink to create at the target, relative to the target directory.
+This can be used to reference files in other sources.
+
+Supported attributes:
+
+* `target` -
+  Content of the symlink.  This is typically a relative path.
+
+
 ## References
 
 - [In-depth example](http://tech.ingolf-wagner.de/nixos/krops/) by [Ingolf Wagner](https://ingolf-wagner.de/)
 
+
 ## Communication
 
 Comments, questions, pull-requests, etc. are very welcome, and can be directed