docs: added Endpoint conventions

Author: mvegter

docs/CONTRIBUTING.md

@@ -1,6 +1,7 @@
 # Contributing to AliceO2 Bookkeeping
 We would love for you to contribute to *AliceO2 Bookkeeping* and help make it even better than it is today! As a contributor, here are the guidelines we would like you to follow:
 - [Coding conventions](#coding-conventions)
+- [Endpoint conventions](#endpoint-conventions)
 - [Commit Message Guidelines](#commit-message-guidelines)
 
 ## Coding conventions
@@ -9,6 +10,22 @@ To ensure consistency throughout the source code, keep these rules in mind as yo
 - All endpoints must be tested by one or more specs (e2e-tests).
 - All code must be formatted. An automated formatter is available (`npm run lint:fix`).
 
+## Endpoint conventions
+A **resource can be a singleton or a collection**. For example, "`customers`" is a collection resource and "`customer`" is a singleton resource (in a banking domain). We can identify "`customers`" collection resource using the URI "`customers`". We can identify a single "`customer`" resource using the URI "`/customers/{customerId}`".
+
+A **resource may contain sub-collection resources** also. For example, sub-collection resource "`accounts`" of a particular "`customer`" can be identified using the URI "`/customers/{customerId}/accounts`" (in a banking domain). Similarly, a singleton resource "`account`" inside the sub-collection resource "`accounts"` can be identified as follows: "`/customers/{customerId}/accounts/{accountId}`".
+
+### Example
+```
+GET   /orders          <---> orders
+POST  /orders          <---> orders.push(data)
+GET   /orders/1        <---> orders[1]
+PUT   /orders/1        <---> orders[1] = data
+PATCH /orders/1        <---> orders[1] = { ...orders[1], ...data }
+GET   /orders/1/lines  <---> orders[1].lines
+POST  /orders/1/lines  <---> orders[1].lines.push(data)
+```
+
 ## Commit Message Guidelines
 We have very precise rules over how our git commit messages can be formatted. This leads to **more readable messages** that are easy to follow when looking through the **project history**. But also, we use the git commit messages to **generate the change log**.