Add brief instruction how to implement a new check (#365)

CONTRIBUTING.md

@@ -19,3 +19,47 @@ Set `TEST_PG_VERSION` environment variable to use any of other available Postgre
 TEST_PG_VERSION=11.20-alpine
 ```
 List of all available PostgreSQL versions can be found [here](https://hub.docker.com/_/postgres).
+
+## Implementing a new check
+
+### Write a new SQL query
+
+Each database structure check starts with an SQL query to the pg_catalog.
+
+1. SQL queries for checks are located in a separate repository https://github.com/mfvanek/pg-index-health-sql
+2. That repository is pulled into the current project as a git submodule
+3. [SQLFluff](https://github.com/sqlfluff/sqlfluff) is used as a linter for all sql queries
+4. All requests must be schema-aware (see [example](https://github.com/mfvanek/pg-index-health-sql/blob/6a5b823d2f86f3fed946f073de93a20245b8d312/sql/duplicated_indexes.sql#L23))
+
+### Extend domain model (if needed)
+
+pg-index-health is a [multimodule Gradle](https://docs.gradle.org/current/userguide/multi_project_builds.html) project.  
+Domain model is located in a [pg-index-health-model](https://github.com/mfvanek/pg-index-health/tree/master/pg-index-health-model).
+All domain classes should be minimalistic and well-defined.
+They should include enough information to generate corrective SQL migrations via [pg-index-health-generator](https://github.com/mfvanek/pg-index-health/tree/master/pg-index-health-generator).
+
+### Add the code for a new check
+
+All checks can be divided into 2 parts:
+1. Runtime checks (those that make sense to run only on a production database)
+2. Static checks (which can be run in tests on an empty database)
+
+Runtime checks usually require aggregation of data from all nodes in the cluster.
+Because of this, it became necessary to create [our own abstraction over the database connection](https://github.com/mfvanek/pg-index-health/tree/master/pg-index-health-jdbc-connection).
+
+#### Execution on a specific host
+
+* Checks on a specific host https://github.com/mfvanek/pg-index-health/tree/master/pg-index-health/src/main/java/io/github/mfvanek/pg/checks/host
+
+Implement a new class extending [AbstractCheckOnHost](https://github.com/mfvanek/pg-index-health/blob/master/pg-index-health/src/main/java/io/github/mfvanek/pg/checks/host/AbstractCheckOnHost.java).
+
+#### Execution on the cluster
+
+* Checks on the cluster https://github.com/mfvanek/pg-index-health/tree/master/pg-index-health/src/main/java/io/github/mfvanek/pg/checks/cluster
+
+Implement a new class extending [AbstractCheckOnCluster](https://github.com/mfvanek/pg-index-health/blob/master/pg-index-health/src/main/java/io/github/mfvanek/pg/checks/cluster/AbstractCheckOnCluster.java).
+
+### Write proper tests
+
+* Your code must be 100% covered.
+* Mutation tests via [pitest](https://pitest.org/) should work.