Update benchmark running instructions. (#9621)

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Add some details on how to run the benchmarks (create-pages, and markdown). I was looking for performance comparisons for a node optimization and it was not initially obvious how to run it. I since figured it out.  Here for example is a 3% performance improvement through the optimization in node.

NEW
```
success Building static HTML for pages — 0.613 s — 5001/5001 10403.89 pages/second
info Done building in 10.092 sec
```
OLD
```
success Building static HTML for pages — 0.631 s — 5001/5001 10071.25 pages/second
info Done building in 10.353 sec
```

benchmarks/create-pages/README.md

@@ -3,3 +3,10 @@
 Stress tests creating lots of tiny pages.
 
 Defaults to building a site with 5k pages. Set the `NUM_PAGES` environment variable to change that e.g. `NUM_PAGES=25000 gatsby build`
+
+# Running the benchmark
+First install node modules required by package.json. This is needed only one time. Then run the build
+```bash
+npm install
+npm run build
+```

benchmarks/markdown/README.md

@@ -3,3 +3,10 @@
 Stress tests creating lots of pages rendered from Markdown.
 
 Defaults to building a site with 5k markdown pages. Set the `NUM_PAGES` environment variable to change that e.g. `NUM_PAGES=25000 gatsby build`
+
+# Running the benchmark
+First install node modules required by package.json. This is needed only one time. Then run the build
+```bash
+npm install
+npm run build
+```