doc: add Continuous Integration

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

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+# Continuous Integration (CI)
+
+Shadow runs a CI workflow every time a pull-request (PR) is updated. This
+workflow contains several checks to assure the quality of the project, and
+only pull-requests with green results are merged.
+
+## Build & install
+
+The project is built & installed on Ubuntu, Alpine, Debian and Fedora. The last
+three distributions are built & installed on containers, and the workflow can
+be triggered locally by following the instructions specified in the
+[Build & install](build_install.md#containers) page.
+
+## System tests
+
+The project is tested on Ubuntu. For that purpose it is built & installed in
+this distribution in a VM. You can run this step locally by following the
+instructions provided in the [Tests](tests.md#system-tests) page.
+
+## Static code analysis
+
+C and shell static code analysis is also executed. For that purpose
+[CodeQL](https://codeql.github.com/) and
+[Differential ShellCheck](https://github.com/marketplace/actions/differential-shellcheck)
+are used.