Add Powershell commands to README.md (facebook#3515)

Author: Gua-naiko-che

packages/react-scripts/template/README.md

@@ -932,6 +932,12 @@ set "REACT_APP_SECRET_CODE=abcdef" && npm start
 
 (Note: Quotes around the variable assignment are required to avoid a trailing whitespace.)
 
+#### Windows (Powershell)
+
+```Powershell
+($env:REACT_APP_SECRET_CODE = "abcdef") -and (npm start)
+```
+
 #### Linux, macOS (Bash)
 
 ```bash
@@ -1179,6 +1185,12 @@ To do this, set the `HTTPS` environment variable to `true`, then start the dev s
 set HTTPS=true&&npm start
 ```
 
+#### Windows (Powershell)
+
+```Powershell
+($env:HTTPS = $true) -and (npm start)
+```
+
 (Note: the lack of whitespace is intentional.)
 
 #### Linux, macOS (Bash)
@@ -1527,6 +1539,16 @@ set CI=true&&npm run build
 
 (Note: the lack of whitespace is intentional.)
 
+##### Windows (Powershell)
+
+```Powershell
+($env:CI = $true) -and (npm test)
+```
+
+```Powershell
+($env:CI = $true) -and (npm run build)
+```
+
 ##### Linux, macOS (Bash)
 
 ```bash