Add README

This is based on the about.rst documentation.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

README.md

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+# Pungi
+
+*Pungi* is a distribution compose tool.
+
+Composes are release snapshots that contain release deliverables such as:
+
+- installation trees
+    - RPMs
+    - repodata
+    - comps
+- (bootable) ISOs
+- kickstart trees
+    - anaconda images
+    - images for PXE boot
+
+
+## Tool overview
+
+*Pungi* consists of multiple separate executables backed by a common library.
+
+The main entry-point is the `pungi-koji` script. It loads the compose
+configuration and kicks off the process. Composing itself is done in phases.
+Each phase is responsible for generating some artifacts on disk and updating
+the `compose` object that is threaded through all the phases.
+
+*Pungi* itself does not actually do that much. Most of the actual work is
+delegated to separate executables. *Pungi* just makes sure that all the
+commands are invoked in the appropriate order and with correct arguments. It
+also moves the artifacts to correct locations.
+
+
+## Links
+
+- Upstream GIT: https://pagure.io/pungi/
+- Issue tracker: https://pagure.io/pungi/issues
+- Questions can be asked on *#fedora-releng* IRC channel on FreeNode