Update README.md with local environment setup

Closes #3499975 Adds local environment setup instructions to the README

README.md

@@ -27,6 +27,80 @@ Ensure that the c2patool binary is on the system
 - Enter in the path to your certificates directory, if the certificate and key are in the $C2PA_SIGN_CERT and $C2PA_PRIVATE_KEY environment variables they will be automatically detected.
 - Enter in the path to the c2patool binary (if it is located in the $PATH it will be automatically detected).
 
+## LOCAL ENVIRONMENT SETUP
+
+You can check the status of the "c2patool" and the "C2PA Signing Certificate"
+from the Drupal Status report. If either of these show a warning or error you
+may need to do some additional setup/configuration.
+
+### c2patool
+
+The `c2patool` will need to be executable from inside your local environment.
+Inside your container configuration, you will want to make sure that the
+following 2 commands are run to install `rust` then the `c2patool`.
+```shell
+/usr/bin/curl https://sh.rustup.rs -sSf | sh -s -- -y
+${HOME}/.cargo/bin/cargo install c2patool --version=^0.9.1 --root=/usr/local --bin
+```
+If your container has `c2patool` installed successfully, you will see the
+version in the status report, similar to:
+---
+***c2patool***
+- /usr/local/bin/c2patool(0.9.12)
+- Executable found at /usr/local/bin/c2patool (0.9.12)
+---
+
+### C2PA Signing Certificate
+You can download the `es256` certificate chain and key from the:
+[c2patool sample folder in GitHub](https://github.com/contentauth/c2patool/tree/main/sample)
+
+Save these to a folder that you can access inside your container, for example
+inside "sites/default/files".  The files need to have the same name, one with
+the ".key" extension and one with the ".pem".  This constitutes the key "pair".
+
+After saving the files, you would have a file structure similar to:
+```text
+├── sites
+    ├── default
+        ├── files
+            ├── shared
+                ├── signing_certs
+                    ├── es256.key
+                    └── es256.pem
+```
+Then inside of your `settings.local.php` file, you will define the config
+override:
+```php
+$config['c2pa_sign.settings']['certificate_file_directory'] = 'sites/default/files/shared/signing_certs/';
+```
+(note this needs a trailing slash)
+
+Then inside of your status report you should see information similar to:
+
+---
+***C2PA Signing Certificate***
+
+sites/default/files/shared/signing_certs/es256.pem\
+CERTIFICATE DETAILS\
+Certificate File: sites/default/files/shared/signing_certs/es256.pem\
+Certificate usage count: 0\
+Valid Not Before: Fri, 10 Jun 2022 18:46:40 +0000\
+Valid Not After: Mon, 26 Aug 2030 18:46:40 +0000\
+Signing Algorithm: ecdsa-with-SHA256
+- Issuer ...
+- Subject ...
+---
+
+### c2pa_sign_aws_kms
+Inside of a development environment, you will most likely NOT have AWS signing
+keys, so you will see an error on the "Content Credentials" screen, similar to:
+```text
+Error connecting to AWS KMS to get available keys: Error executing "ListAliases" on "https://kms.us-gov-west-1.amazonaws.com";
+```
+You can disable the `c2pa_sign_aws_kms` locally to avoid this message, but the
+c2patool should function correctly despite the message.  If you disable this
+module, please make sure not to let that config get upstream.
+
 ## MAINTAINERS
 
 Current maintainers for Drupal 10: