Add dev doc about worker unallocated distribution (#5041)

Co-authored-by: Sam Doran <[email protected]>

docs/distributing_cost.md

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+# Distributing Cost
+Our cost model allows user to distribute the their costs bassed of the CPU or memory usage of their user projects.
+
+## Worker Unallocated Costs:
+
+### How We Calculate
+
+During data processing we add in additional rows to represent the [worker unallocated](https://github.com/project-koku/koku/blob/main/koku/masu/database/trino_sql/reporting_ocpusagelineitem_daily_summary.sql) cost. These rows are added under the project name `Worker unallocated` during the OCP daily summary table creation. Formula:
+```
+unallocated_worker_cost = capacity - effective usage
+```
+The effective usage is the `max(request, usage)`, since users can have a higer usage than their request.
+
+### How We Distribute
+
+One of the features of our cost models is to distribute the cost associated with the `Worker unallocated` project to the user projects with this algorithm:
+```
+distributed_cost = (user_project_usage / usage_of_all_user_projects) * worker_unallocated_cost
+```
+
+For example, there are worker unallocated cost of 100 dollars and two user projects, A and B. Project A has a usage of 25 and project B has a usage of 75. Using the algorithm above to distribute the worker unallocated cost:
+
+```
+worker_unallocated_cost = 100
+Project's A distributed_cost = (25 / sum(25 + 75)) * 100 = 25
+Project's B distributed_cost = (75 / sum(25 + 75)) * 100 = 75
+```
+
+Project A gets an additional 25 dollars and Project B gets an additional 75 dollars due to distributing the worker unallocated cost.
+
+**How we deduplicate**
+
+The `Worker unallocated` project that we created earlier in the *How We Calculate* section still holds the original cost. Using the example above this would be our original 100 dollars. In order to deduplicate costs for the `Worker unallocated` project we need to subtract the unallocated costs:
+
+```
+distributed_cost = 0 - worker_uanllocated_cost
+```
+
+Working with our same example:
+
+```
+Worker unallocated project's distributed_cost = 0 - 100
+```