Add post install script to test if SPFx version 1.12 - 1.14

Show's message that updates are needed & instructions are in the getting started guide

package.json

@@ -51,7 +51,8 @@
         "serve": "tsc -p ./debug/serve/tsconfig.json && node ./build/server/debug/serve/plumbing/run.js",
         "start": "npm run serve",
         "test": "tsc -p ./test/tsconfig.json && mocha --verbose --logging",
-        "test-build": "tsc -p ./test/tsconfig.json"
+        "test-build": "tsc -p ./test/tsconfig.json",
+        "postinstall": "node tools/postinstall/script.cjs"
     },
     "repository": {
         "type": "git",
@@ -86,4 +87,4 @@
         "url": "https://github.com/pnp/pnpjs/issues"
     },
     "homepage": "https://github.com/pnp/pnpjs"
-}
+}

tools/postinstall/script.cjs

@@ -0,0 +1,19 @@
+const fs = require("fs");
+const projectRoot = process.env.INIT_CWD;
+const packageLoc = `${projectRoot}\\package.json`;
+const packageFile = fs.readFileSync(packageLoc, "utf8");
+const packageJSON = JSON.parse(packageFile);
+if (packageJSON.dependencies != null) {
+    const spfxVersion = packageJSON.dependencies["@microsoft/sp-core-library"];
+    if (spfxVersion != null) {
+        const spfxVersionFloat = parseFloat(spfxVersion);
+        if (spfxVersionFloat > 1.11 && spfxVersionFloat < 1.15) {
+            console.log("");
+            console.log("\x1b[43m%s\x1b[0m", " PnPjs WARNING ");
+            console.log("\x1b[33m%s\x1b[0m", "  The version of SPFx you are using requires an update to work with PnPjs. Please make sure to follow the getting started instructions to make the appropriate changes. ➡ https://pnp.github.io/pnpjs/getting-started/#spfx-version-1121-later");
+            console.log("");
+        }
+    }
+} else {
+    console.log("Package has no dependencies");
+}