Add developer docs

Adding docs explaining how different parts are working in conjuction. In this initial attemp I am adding inputs/outputs contract is
controllers.

docs/developers/README.md

@@ -0,0 +1,54 @@
+## Developer docs
+
+This document is aimed at helping maintainers/developers of project understand the complexity.
+
+### How are resources shared between tasks
+
+Pipelineruns uses PVC to share resources between tasks. PVC volume is mounted on path `/pvc` by Pipelinerun. 
+
+* If resource in a task is declared as output then taskun controllers add step to copy each output resource to directory path `/pvc/task_name/resource_name`.
+
+* If input resource includes `providedBy` condition then container definition to copy from PVC to directory from `/pvc/previous_task/resource_name`.
+
+### How are inputs handled?
+
+Input resources like source code(git) or artifacts are dumped at path `/workspace/`. Resource definition in task can have custom target directory. If `targetPath` is mentioned then controllers have to be responsible for adding container definition to create directories and pulling down the versioned artifacts into that directory.
+
+### How are outputs handled?
+
+Output resources like source code(git) or artifacts(storage resource) are expected in directory path `/workspace/output/resource_name`. 
+
+* If resource has output "action" like upload to blob storage, then container step is added for this action.
+* If there is PVC volume present(taskrun holds owner reference to pipelinerun) then copy step is added as well. 
+    * Copy step includes resource being copied to PVC to path `/pvc/task_name/resource_name` from `/workspace/output/resource_name` if resource is not declared in input resource like below example.
+
+    ```yaml
+    kind: Task
+    metadata:
+    name: get-gcs-task
+    namespace: default
+    spec:
+    outputs:
+        resources:
+        - name: gcs-workspace
+        type: storage
+    ```
+    
+    * Copy step includes resource being copied to PVC to path `/pvc/task_name/resource_name` from `/workspace/random-space/` if resource is declared in input resource like below example.
+
+    ```yaml
+    kind: Task
+    metadata:
+    name: get-gcs-task
+    namespace: default
+    spec:
+    inputs:
+        resources:
+        - name: gcs-workspace
+        type: storage
+        targetPath: random-space
+    outputs:
+        resources:
+        - name: gcs-workspace
+        type: storage
+    ```