Add Warning Signs manual entry

src/main/resources/assets/immersiveengineering/manual/en_us/warning_signs.txt

@@ -1,4 +1,6 @@
 Warning Signs
 Warning: Excessive signage ahead
-A witty text about signs goes here, explaining why they're important and that you can put them on banners
-<&sign_recipes>
+Factories and warehouses are often grimy, unsafe, and riddled with chemicals and steam clouding the air.<br>
+If reducing these hazards is inefficient, newly standardized Warning Signs can be used to warn of dangers instead.
+<&sign_recipes>Warning signs can be crafted from any metal backing plate and yellow dye, to make sure the signs are visible on gloomy factory floors.<br>
+Signs can be placed on any vertical block face, but cannot be placed on the floor or on the ceiling.