docs: add storage/addon-block documentation draft

docs/storage/addon-block.md

@@ -0,0 +1,40 @@
+# Add-on Block
+
+Fixed storage which stores an immutable binary-search tree of items.
+Each item consists of a pair of pointers to binary data of arbitrary length.
+These two pointers are interpreted as a `key` and a `value`.
+Since items contain only pointers, multiple instances of data are stored just once.
+
+Block contains 3 logical parts:
+a) header (256 bytes)
+b) binary-search tree (variable size)
+c) item data (variable size)
+
+## Header
+
+| offset | length   | name         | description  |
+|-------:|---------:|--------------|--------------|
+| 0x0000 | 4        | magic        | magic `TRAB` |
+| 0x0004 | 64       | sig          | signature of the whole blob (except magic and sig) |
+| 0x0044 | 4        | tree_count   | number of elements in the tree |
+| 0x0048 | 4        | items_size   | length of the item section |
+| 0x004A | 182      | reserved     | - |
+
+## Binary-Search Tree
+
+Each node of the tree has the following structure:
+
+| offset | length   | name         | description  |
+|-------:|---------:|--------------|--------------|
+| ...    | 2        | key_offset   | key   offset (divided by 4) |
+| ...    | 2        | value_offset | value offset (divided by 4) |
+
+# Item Data
+
+Each item has the following structure:
+
+| offset | length   | name         | description  |
+|-------:|---------:|--------------|--------------|
+| ...    | 2        | item_len     | item (key or value) length |
+| ...    | 2        | item_flags   | item (key or value) flags |
+| ...    | item_len | item_data    | item (key or value) data (padded to multiple of 4) |