docs: some devel intro

devel.md

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+# Getting started
+
+1. Install/configure venv using requirements.txt.
+2. Do not forget to update your python version in commands below.
+3. All ansible-test invocations should happen in `ansible_collections/serverscom/sc_api`.
+   This is ansible-test requirement.
+4. Installing test dependencies: add `--requirements` to ansible-test during normal calls.
+
+Running sanity checks:
+
+```
+ansible-test sanity --requirements --python 3.11
+```
+
+unit tests:
+
+```
+ansible-test units --requirements --python 3.11
+```
+(if you get odd errors from pytest, don't forget --requirements)
+
+Both unit and sanity checks can work without secrets.
+
+# Integration tests
+
+To run integration tests you need to generate your own config at
+`ansible_collections/serverscom/sc_api/tests/integration/integration_config.yml`
+(this location is odd, but it's the single place where ansible-test permits any side
+causes into test playbooks)
+
+See example in `integration_config.yml.template`.
+
+Please, pay attention,
+you need at least 3 baremetal servers to run tests for baremetal modules.
+
+The main secret you get is SC_TOKEN, which is token to access servers.com API.
+
+At the time of writing you could get one from here: https://portal.servers.com/profile/api-tokens
+
+Please be careful, some tests will cause significant financial spendings. They also will reinstall baremetal servers, so you should dedicate servers for those testing exlusively.