Update README.md for static-exporter to add explicit instructions to update the httpd.conf

Author: Pokom

static-exporter/README.md

@@ -43,3 +43,22 @@ local static_exporter = import 'github.com/grafana/jsonnet-libs/static-expoter/m
     ]),
 }
 ```
+
+## Updating httpd.conf
+
+There is a default httpd.conf that was added to this library.
+It was generated by running the following:
+
+```
+docker run --rm httpd:2.4 cat /usr/local/apache2/conf/httpd.conf httpd.conf
+```
+
+If there is a downstream change that requires updating this config file, run the above command and then add the following snippet to the `<Directory "/usr/local/apache2/htdocs"` block:
+
+```
+  <IfModule mod_headers.c>
+        Header set Content-Type: "text/plain; version=0.0.4"
+    </IfModule>
+```
+
+This change adds a Header to the requests that enables Prometheus 3.x to scrape the static exporter.