LTS-5452: README step to set credentials as env vars

.gitignore

@@ -0,0 +1,3 @@
+# BrowserStack CLI binary, downloaded into the project root (see README)
+browserstack-cli
+browserstack-cli.exe

README.md

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 # browserstack-playwright-robot-load-testing-sample
 Sample repository for Playwright framework to do load testing
 
+## Set your credentials
+
+The BrowserStack CLI reads your credentials from environment variables. Copy your username and access key from [Account Settings](https://www.browserstack.com/accounts/settings) and export them in the shell you run the test from:
+
+On macOS or Linux (bash or zsh), run the lines below. The second one prompts for the key without echoing it, so the key stays out of your shell history:
+
+```sh
+export BROWSERSTACK_USERNAME="<your-username>"
+printf 'BrowserStack access key: '; read -rs BROWSERSTACK_ACCESS_KEY; echo; export BROWSERSTACK_ACCESS_KEY
+```
+
+On Windows, run these in PowerShell. The second line prompts for the key in the same way:
+
+```powershell
+$env:BROWSERSTACK_USERNAME = "<your-username>"
+$env:BROWSERSTACK_ACCESS_KEY = [System.Net.NetworkCredential]::new('', (Read-Host -AsSecureString 'BrowserStack access key')).Password
+```
+
+These variables last only for the current terminal session. If the CLI reports an authentication error, check that both are set in the terminal you run it from.
+
+**Never commit your credentials.** Keep your access key out of `browserstack-load.yml` and every other file you push. If a key is ever exposed, regenerate it from Account Settings.
+
+## Run the sample
+
+1. Download the BrowserStack CLI for your operating system. The download is a zip: extract it and place the `browserstack-cli` binary (`browserstack-cli.exe` on Windows) in the root of this project. On macOS or Linux, also run `chmod +x browserstack-cli`, since some unzip tools drop the executable bit.
+   - [macOS x86](https://load-api.browserstack.com/api/v1/binary?os=macos&arch=x64)
+   - [macOS ARM](https://load-api.browserstack.com/api/v1/binary?os=macos&arch=arm64)
+   - [Windows x86](https://load-api.browserstack.com/api/v1/binary?os=win&arch=x64)
+   - [Linux x86](https://load-api.browserstack.com/api/v1/binary?os=linux&arch=x64)
+   - [Linux ARM](https://load-api.browserstack.com/api/v1/binary?os=linux&arch=arm64)
+2. On macOS and Windows, check that the binary is signed by BrowserStack before running it. On macOS the command must succeed: it only passes if an Apple-issued certificate for BrowserStack's team (`YQ5FZQ855D`) signed the binary; on Windows, `Status` must be `Valid` with BrowserStack as the signer. If the check fails, do not run the binary. There is no signature check for the Linux build.
+
+   ```sh
+   codesign --verify --strict -R '=anchor apple generic and certificate leaf[subject.OU] = "YQ5FZQ855D"' ./browserstack-cli
+   ```
+
+   ```powershell
+   # Windows (PowerShell)
+   Get-AuthenticodeSignature .\browserstack-cli.exe | Format-List Status, SignerCertificate
+   ```
+3. Start the test:
+
+   ```sh
+   ./browserstack-cli load run
+   ```
+
+   ```powershell
+   # Windows (PowerShell)
+   .\browserstack-cli.exe load run
+   ```
+
 ## Notes before you adapt this sample
 
 - **Dependencies are hash-pinned.** `requirements.txt` is a lock with a hash for every package, so pip refuses anything that doesn't match. Edit the direct dependencies in `requirements.in`, then regenerate the lock: