add warning when GATSBY_GITHUB_TOKEN is not present

* closes  #372

.env.example

@@ -1 +1 @@
-#GATSBY_GITHUB_TOKEN=
+GATSBY_GITHUB_TOKEN=xxx

README.md

@@ -158,6 +158,7 @@ git clone git@github.com:kremalicious/portfolio.git
 cd portfolio/
 
 # GATSBY_GITHUB_TOKEN is required for some parts
+# See https://docs.github.com/en/github/authenticating-to-github/creating-a-personal-access-token
 cp .env.sample .env
 vi .env
 

gatsby-config.js

@@ -8,6 +8,16 @@ const { name, website } = resume.basics
 
 require('dotenv').config()
 
+if (
+  !process.env.GATSBY_GITHUB_TOKEN ||
+  process.env.GATSBY_GITHUB_TOKEN === 'xxx'
+) {
+  throw Error(`
+      ⚠️  A GitHub token as GATSBY_GITHUB_TOKEN is required to build some parts of the blog.
+      ⚠️  Check the README https://github.com/kremalicious/portfolio#-development.
+  `)
+}
+
 module.exports = {
   siteMetadata: {
     siteUrl: `${website}`