[docs] Describe mapping info contribution

Author: dimkarakostas

docs/contribute.md

@@ -4,4 +4,36 @@ You can contribute to the tool by adding support for a ledger, updating the
 mapping process for an existing ledger, or adding a new metric. In all cases,
 the information should be submitted via a GitHub PR.
 
-...
+## Add support for ledgers
+
+You can add support for a ledger that is not already supported as follows.
+
+### Mapping information
+
+In the directory `mapping_information/`, there exist two folders: `addresses`
+and `special_addresses`.
+
+`addresses` contains information about the owner or manager of an address. This
+information should be publicly available and verifiable, for example it may come
+from a public explorer, social media or forum posts, articles, etc. Each file in
+this folder is named `<project_name>.json` (for the corresponding ledger) and
+contains a dictionary where the key is the address and the value is a dictionary
+with the following information:
+(i) the name of the entity (that controls the address);
+(ii) the source of the information (e.g., an explorer's URL);
+(iii) (optional) a boolean value `is_contract` (if omitted then it is assumed false);
+(iv) (optional) `extra_info` that might be relevant or interesting (not used for
+the analysis).
+
+`special_addresses` contains information about addresses that should be treated
+specially, e.g., excluded from the analysis. This includes burn addresses,
+protocol-related addresses (e.g., Ethereum's staking contract), treasury
+addresses, etc. Here each file is named `<project_name>.json` and contains a
+list of dictionaries with the following information:
+(i) the address;
+(ii) the source of the information;
+(iii) `extra_info` which describes the reason why the address is special.
+
+To contribute mapping information you can either update an existing file, by
+changing and/or adding some entries, or create a new file for a newly-supported
+ledger.