Add conf & overlay readmes

Author: JedMeister

conf/turnkey.d/README.md

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+Common TurnKey configuration scripts
+====================================
+
+These configuration scripts will be applied to all TurnKey appliances.
+
+Architechture family specific conf scripts
+------------------------------------------
+
+Architecture family specific conf scripts can be put in a relevant
+subdirectory here.
+
+Supported architechture families are:
+- x86 (amd64 aka x86_64)
+- arm (arm64 aka aarch64)
+
+E.g. to run a new 'amd64-only' configuration script on amd64 builds (only):
+
+```
+cd common
+mkdir -p conf/turnkey.d/x86.d
+cat > conf/turnkey.d/x86.d/amd64-only <<EOF
+# configuration script only for amd64 builds
+#
+# ...
+EOF
+chmod +x conf/turnkey.d/x86.d/amd64-only
+```

overlays/turnkey.d/README.md

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+Common TurnKey overlays
+=======================
+
+These overlays will be applied to all TurnKey appliances.
+
+Architechture family specific overlays
+--------------------------------------
+
+Architecture family specific overlays can be put in a relevant
+subdirectory here.
+
+Supported architechture families are:
+- x86 (amd64 aka x86_64)
+- arm (arm64 aka aarch64)
+
+E.g. to add a new /etc/some-app/amd64-only.conf overlay that should only be
+added to amd64 builds:
+
+```
+cd common
+mkdir -p overlays/turnkey.d/x86.d/etc/some-app
+cat > overlays/turnkey.d/x86.d/etc/some-app/amd64-only.conf <<EOF
+# config only for amd64 builds
+#
+# ...
+EOF
+```