Add markdown files for documentation

projects.xml and declarative-projects.xml were merged with xmllint, and
then I ran that to convert files
for i in *.xml; do pandoc -s -f docbook -t markdown $i -o ${i/xml/md}; done

doc/manual/src/hacking.md

@@ -0,0 +1,28 @@
+Hacking
+=======
+
+This section provides some notes on how to hack on Hydra. To get the
+latest version of Hydra from GitHub:
+
+    $ git clone git://github.com/NixOS/hydra.git
+    $ cd hydra
+
+To build it and its dependencies:
+
+    $ nix-build release.nix -A build.x86_64-linux
+
+To build all dependencies and start a shell in which all environment
+variables (such as PERL5LIB) are set up so that those dependencies can
+be found:
+
+    $ nix-shell
+
+To build Hydra, you should then do:
+
+    [nix-shell]$ ./bootstrap
+    [nix-shell]$ configurePhase
+    [nix-shell]$ make
+
+You can run the Hydra web server in your source tree as follows:
+
+    $ ./src/script/hydra-server