Update README of all three techdocs packages/plugins

packages/techdocs-common/README.md

@@ -2,6 +2,46 @@
 
 Common functionalities for TechDocs, to be shared between techdocs plugins and techdocs-cli
 
+This package is used by `techdocs-backend` to serve docs from different types of publishers (Google GCS, Local, etc.).
+It is also used to build docs and publish them to storage, by both `techdocs-backend` and `techdocs-cli`.
 ## Usage
 
-TODO: List supported APIs
+Create a preparer instance from the [preparers available](/packages/techdocs-common/src/stages/prepare) at which takes an Entity instance.
+Run the [docs generator](/packages/techdocs-common/src/stages/generate) on the prepared directory.
+Publish the generated directory files to a [storage](/packages/techdocs-common/src/stages/publish) of your choice.
+
+Example:
+```js
+async () => {
+  const preparedDir = await preparer.prepare(entity);
+
+  const parsedLocationAnnotation = getLocationForEntity(entity);
+  const { resultDir } = await generator.run({
+    directory: preparedDir,
+    dockerClient: dockerClient,
+    parsedLocationAnnotation,
+  });
+
+  await publisher.publish({
+    entity: entity,
+    directory: resultDir,
+  });
+}
+```
+
+## Features
+
+Currently the build process is split up in these three stages.
+
+- Preparers
+- Generators
+- Publishers
+
+Preparers read your entity data and creates a working directory with your documentation source code. For example if you have set your `backstage.io/techdocs-ref` to `github:https://github.com/backstage/backstage.git` it will clone that repository to a temp folder and pass that on to the generator.
+
+Generators takes the prepared source and runs the `techdocs-container` on it. It then passes on the output folder of that build to the publisher.
+
+Publishers gets a folder path from the generator and publish it to your storage solution. Read documentation to know more about configuring storage solutions.
+http://backstage.io/docs/features/techdocs/configuration
+
+Any of these can be extended. We want to extend our support to most of the storage providers (Publishers) and source code host providers (Preparers).