update contributing guide and ui-tests scripts (#1668)

- Updated ./docs/CONTRIBUTION.md to clarify how to use command bot and
update ui-tests locally or in PR
- Moved update-ui-tests.sh to a root of project to run them along for
substrate and polkadot together

---------

Co-authored-by: Mira Ressel <mira@parity.io>

docs/CONTRIBUTING.md

@@ -143,4 +143,18 @@ UI tests are used for macros to ensure that the output of a macro doesn’t chan
 These UI tests are sensible to any changes in the macro generated code or to switching the rust stable version.
 The tests are only run when the `RUN_UI_TESTS` environment variable is set. So, when the CI is for example complaining
 about failing UI tests and it is expected that they fail these tests need to be executed locally.
-To simplify the updating of the UI test output there is the `.maintain/update-rust-stable
+To simplify the updating of the UI test output there is a script
+- `./scripts/update-ui-tests.sh`   to update the tests for a current rust version locally
+- `./scripts/update-ui-tests.sh 1.70` # to update the tests for a specific rust version locally
+
+Or if you have opened PR and you're member of `paritytech` - you can use command-bot to run the tests for you in CI:
+- `bot update-ui` - will run the tests for the current rust version
+- `bot update-ui latest --rust_version=1.70.0` - will run the tests for the specified rust version
+- `bot update-ui latest -v CMD_IMAGE=paritytech/ci-unified:bullseye-1.70.0-2023-05-23 --rust_version=1.70.0` -
+will run the tests for the specified rust version and specified image
+
+## Command Bot
+
+If you're member of **paritytech** org - you can use command-bot to run various of common commands in CI:
+
+Start with comment in PR: `bot help` to see the list of available commands.