Doc: use custom sharding key to calculate bucket id

Describe functionality and current limitations (#212 and #213) with
custom sharding key in CHANGELOG and README.

Closes #166

CHANGELOG.md

@@ -24,6 +24,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * `crud.len()` function to calculate the number of tuples
   in the space for memtx engine and calculate the maximum
   approximate number of tuples in the space for vinyl engine.
+* CRUD operations calculates bucket id automatically using sharding
+  key specified with DDL schema or in `_ddl_sharding_key` space.
+  NOTE: CRUD methods delete(), get() and update() requires that sharding key
+  must be a part of primary key.
 
 ## [0.8.0] - 02-07-21
 

README.md

@@ -54,10 +54,58 @@ crud.unflatten_rows(res.rows, res.metadata)
 
 * A space should have a format.
 * By default, `bucket_id` is computed as `vshard.router.bucket_id_strcrc32(key)`,
-  where `key` is the primary key value.
-  Custom bucket ID can be specified as `opts.bucket_id` for each operation.
-  For operations that accepts tuple/object bucket ID can be specified as
-  tuple/object field as well as `opts.bucket_id` value.
+  where `key` is a value of sharding key. Sharding key can be set with [DDL
+  schema](https://github.com/tarantool/ddl#input-data-format) (see
+  `sharding_key` option) or in a space `_ddl_sharding_key`. By default sharding
+  key is a primary key. Custom bucket ID can be specified as `opts.bucket_id`
+  for each operation. For operations that accepts tuple/object bucket ID can be
+  specified as tuple/object field as well as `opts.bucket_id` value.
+  NOTE: CRUD methods delete(), get() and update() requires that sharding key
+  must be a part of primary key.
+
+**Sharding key**
+
+*Sharding key* is a set of tuple field values used for calculation *bucket ID*.
+*Bucket ID* determines which replicaset stores certain data. Function that used
+for calculation bucket ID is named *sharding function*.
+
+By default CRUD calculates bucket ID using primary key, it happen automatically
+and doesn't require any actions from user side. User can calculate bucket ID on
+outside and pass it as an option to CRUD method (see below).
+
+Users who don't want to use primary key as a sharding key may set custom
+sharding key definition as a part of DDL schema or manually to the space
+`_ddl_sharding_key` used by DDL module too (for both cases consider a DDL
+module documentation). As soon as sharding key for a certain space is available
+in `_ddl_sharding_key` space CRUD will use it for bucket ID calculation
+automatically.
+
+Table below describe what operations supports custom sharding key:
+
+| CRUD method                      | Added sharding key support |
+| -------------------------------- | -------------------------- |
+| `get()`                          | Yes                        |
+| `insert()` / `insert_object()`   | Yes                        |
+| `delete()`                       | Yes                        |
+| `replace()` / `replace_object()` | Yes                        |
+| `upsert()` / `upsert_object()`   | Yes                        |
+| `select()` / `pairs()`           | Yes                        |
+| `update()`                       | Yes                        |
+| `upsert()` / `upsert_object()`   | Yes                        |
+| `replace() / replace_object()`   | Yes                        |
+| `min()` / `max()`                | No (not required)          |
+| `cut_rows()` / `cut_objects()`   | No (not required)          |
+| `truncate()`                     | No (not required)          |
+| `len()`                          | No (not required)          |
+
+Current limitations for using custom sharding key:
+
+- It's not possible to update sharding keys automatically when schema is
+updated on storages, see [#212](https://github.com/tarantool/crud/issues/212).
+However it is possible to do it manually with
+`sharding_key.update_sharding_keys_cache()`.
+- CRUD select may lead map reduce in some cases, see
+[#213](https://github.com/tarantool/crud/issues/213).
 
 ### Insert