feat: call git submodule add git@github.com:dancesWithCycles/manual.git source/manual/

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "source/manual"]
+	path = source/manual
+	url = git@github.com:dancesWithCycles/manual.git

README.md

@@ -1,2 +1,23 @@
 # manual-gen
-generate manual
+Generate the manual.
+
+# Build
+
+## Checkout
+This repository accesses the required `manual` repository as a git submodule.
+Therefore, make sure to check out this repository and included submodules:
+
+```
+git clone --recurse-submodules <git remote repository>
+```
+
+If you cloned this repository initially without submobules,
+you can always update the content of submodules with the git submodule update command.
+Execute the command from the main project directory:
+
+```
+git submodule update --init --recursive
+```
+
+The ```--init``` flag initializes the submodules before the update.
+The ```--recursive``` option searches for nested submodules and ensures they are updated too.

source/manual

@@ -0,0 +1 @@
+Subproject commit 7ebc7cddc760fced4ac44001067d6471a73c823a