doc: document how cucumber tests works

integration-tests/README.md

+# Duniter-v2s integration tests
+
+## cucumber functionnal tests
+
+We use [cucumber] to be able to describe test cases in human language.
+
+Cucumber is a specification for running tests in a [BDD] (behavior-driven development) style
+workflow.
+
+It assumes involvement of non-technical members on a project and as such provides a human-readable
+syntax for the definition of features, via the language [Gherkin]. A typical feature could look
+something like this:
+
+```gherkin
+Feature: Balance transfer
+
+  Scenario: If alice sends 5 ĞD to Dave, Dave will get 5 ĞD
+    Given alice have 10 ĞD
+    When alice send 5 ĞD to dave
+    Then dave should have 5 ĞD
+```
+
+### create a new functional test
+
+To create a new test case, simply create a new file with a name of your choice in the `/features`
+folder and give it the extension `.feature`.
+
+Read in the sections below which users are available and which operations you can write.
+
+ [Cucumber Rust Book]
+
+### Test users
+
+6 test users are provided:
+
+- alice
+- bob
+- charlie
+- deve
+- eve
+- ferdie
+
+### Currency amounts
+
+Amounts must be expressed as an integer of `ĞD` or `UD`, decimal numbers are not supported.
+If you need more precision, you can express amounts in cents of ĞD (write `cĞD`), or in thousandths
+of UD (write `mUD`).
+
+#### Given
+
+You can give any currency balance to each of the test users, so money will be created ex-nihilo for
+that user. Note that this created money is not included in the monetary mass used to revalue the UD
+amount.
+
+Usage: `{user} have {amount} {unit}`
+
+Example: `alice have 10 ĞD`
+
+#### When
+
+List of possible actions:
+
+- transfer: `alice send 5 ĞD to bob`
+- transfer_ud: `alice send 3 UD to bob`
+- transfer_all: `alice sends all her ĞDs to bob`
+
+#### Then
+
+-  Check that a user has exactly a specific balance
+
+    Usage: `{user} have {amount} {unit}`
+
+    Example: `alice should have 10 ĞD`
+
+### Universal dividend creation
+
+#### Then
+
+-  Check the current UD amount
+
+    Usage: `Current UD amount should be {amount}.{cents} ĞD`
+
+    Example: `Current UD amount should be 10.00 ĞD`
+
+### Contribute to the code that runs the tests
+
+Cucumber is not magic, we have to write code that interprets the Gherkin text and performs the right
+actions accordingly.
+
+The rust code that interprets the Gherkin text is in this file:
+`integration-tests/tests/cucumber_tests.rs`.
+
+To contribute to this section, read the [Cucumber Rust Book].
+
+To interact with the node, we use exclusively RPC requests, the RPC requests are realized in
+functions defined in `integration-tests/tests/common`.
+
+To realize the RPC requests, we use the rust crate [subxt](https://github.com/paritytech/subxt).
+
+
+
+[BDD]: https://en.wikipedia.org/wiki/Behavior-driven_development
+[cucumber]: https://cucumber.io/
+[Cucumber Rust Book]: https://cucumber-rs.github.io/cucumber/current/writing/index.html
+[Gherkin]: https://cucumber.io/docs/gherkin/reference