Update README with instructions on loopinstall with private plugins (#17790)

Author: chainchad

README.md

@@ -44,6 +44,25 @@ regarding Chainlink social accounts, news, and networking.
 
 For the latest information on setting up a development environment, see the [Development Setup Guide](https://github.com/smartcontractkit/chainlink/wiki/Development-Setup-Guide).
 
+### Build Plugins
+
+Plugins are defined in yaml files within the `plugins/` directory. Each plugin file is a yaml file and has a `plugins.` prefix name. Plugins are installed with [loopinstall](https://github.com/smartcontractkit/chainlink-common/tree/main/pkg/loop/cmd/loopinstall).
+
+Some plugins (such as those in `plugins/plugins.private.yaml`) reference private GitHub repositories. To build these plugins, you must have a GITHUB_TOKEN env var set, or preferably use the [gh](https://cli.github.com/manual/gh) GitHub CLI tool to use the [GitHub CLI credential helper](https://cli.github.com/manual/gh_auth_setup-git) like:
+
+```shell
+# Sets up a credential helper.
+gh auth setup-git
+```
+
+Then you can build the plugins with:
+
+```shell
+make install-loopinstall # install loopinstall
+make install-plugins-public # install public plugins
+make install-plugins-private # install private plugins
+```
+
 ### Apple Silicon - ARM64
 
 Native builds on the Apple Silicon should work out of the box, but the Docker image requires more consideration.
@@ -184,19 +203,20 @@ This script will save the `CL_DATABASE_URL` in `.dbenv`
 
 Changes to database require migrations to be run. Similarly, `pull`'ing the repo may require migrations to run.
 After the one-time setup above:
+
 ```
 source .dbenv
 make testdb
 ```
 
 If you encounter the error `database accessed by other users (SQLSTATE 55006) exit status 1`
 and you want force the database creation then use
+
 ```
 source .dbenv
 make testdb-force
 ```
 
-
 7. Run tests:
 
 ```bash
@@ -249,9 +269,11 @@ flowchart RL
     github.com/smartcontractkit/chainlink/core/scripts --> github.com/smartcontractkit/chainlink/v2
 
 ```
+
 The `integration-tests` and `core/scripts` modules import the root module using a relative replace in their `go.mod` files,
 so dependency changes in the root `go.mod` often require changes in those modules as well. After making a change, `go mod tidy`
 can be run on all three modules using:
+
 ```
 make gomodtidy
 ```
@@ -300,8 +322,9 @@ We use [changesets](https://github.com/changesets/changesets) to manage versioni
 Every PR that modifies any configuration or code, should most likely accompanied by a changeset file.
 
 To install `changesets`:
-  1. Install `pnpm` if it is not already installed - [docs](https://pnpm.io/installation).
-  2. Run `pnpm install`.
+
+1. Install `pnpm` if it is not already installed - [docs](https://pnpm.io/installation).
+2. Run `pnpm install`.
 
 Either after or before you create a commit, run the `pnpm changeset` command to create an accompanying changeset entry which will reflect on the CHANGELOG for the next release.