update doc/en structure

doc/en/engine-use-recommendations.md

@@ -0,0 +1,31 @@
+# Engine usage recommendations
+
+## Content naming
+
+### Content packs ID
+
+Content pack identifier requirements:
+- name can consist of Capital letters A-Z, lowercase letters a-z digits 0-9, and underscore '\_' signs. 
+- the first character must not be a digit.
+- name length must be in range \[2, 24\]
+
+### Blocks and items
+
+- blocks and items identifiers follow the same requirements as content-pack ID.
+- `.item` suffix added only to replace auto-generated block item. Example: `base:stone.item` - an item generated for stone block.
+- **caption** field specifying name displayed in inventory UI should not be Capitalized. The engine does it automatically depending on display context.
+
+## Storage
+
+### Content packs data
+
+Settings and other state that supposed to be saved with a world, must be stored in `world:data/pack_id/`. The path should be retrieved by calling a function:
+```lua
+local path = pack.data_file(PACK_ID, "file_name")
+file.write(path, some_data)
+-- writes data to file world:data/PACK_ID/file_name
+```
+PACK_ID is an existing variable containing current content-pack name.
+
+Directory `world:data/PACK_ID` will be created on call `pack.data_file(...)`.
+