Add docs on secrets (#1693)

* add docs on secrets

* Update packages/homepage/content/docs/5-secrets.md

Co-Authored-By: SaraVieira <hey@iamsaravieira.com>

* Update packages/homepage/content/docs/5-secrets.md

Co-Authored-By: SaraVieira <hey@iamsaravieira.com>

* Update packages/homepage/content/docs/5-secrets.md

Co-Authored-By: SaraVieira <hey@iamsaravieira.com>

* Update packages/homepage/content/docs/5-secrets.md

Co-Authored-By: SaraVieira <hey@iamsaravieira.com>

* Apply suggestions from code review

Co-Authored-By: SaraVieira <hey@iamsaravieira.com>

* Update packages/homepage/content/docs/5-secrets.md

Co-Authored-By: SaraVieira <hey@iamsaravieira.com>

packages/homepage/content/docs/5-secrets.md

@@ -0,0 +1,61 @@
+---
+title: "Secrets"
+authors: ["SaraVieira"]
+description: "CodeSandbox allows you to store secrets so you don't share your keys"
+---
+
+## What are secrets?
+
+Secrets are used to hide sensitive information in your application that you don't want the world to see, and are implemented in CodeSandbox using environment variables.
+The most common use case for using secrets is with API keys.
+
+Secrets values will not be transferred between forks and **can only be used in container sandboxes**.
+
+## Adding secrets in CodeSandbox
+
+You can control the secrets in your Server Control Panel, before adding any it should look like this:
+
+![No Secrets](./images/secrets-1.png)
+
+Let's say you want to add a Google Maps API key. You can do it like so:
+
+![Map Secrets](./images/secrets-2.png)
+
+Pro tip: It's always a good practice to name your secrets all in uppercase.
+
+After clicking the "Save Secret" button the secret is added, the sandbox is restarted, and you can see the list of all your secrets above the form.
+
+![Secrets](./images/secrets-3.png)
+
+As said earlier, secrets are environment variables, meaning they are defined on `process.env`. In the example above, we can read the API key from `process.env.GOOGLE_MAPS_API_KEY`.
+
+Like so:
+
+```js
+var http = require('http');
+
+const key = process.env.GOOGLE_MAPS_API_KEY;
+
+http
+  .createServer(function(req, res) {
+    res.write('This your maps key' + key);
+    res.end(); //end the response
+  })
+  .listen(8080);
+```
+
+The sandbox used is [https://codesandbox.io/s/2v6xq474kj](https://codesandbox.io/s/2v6xq474kj)
+
+## Editing secrets in CodeSandbox
+
+Our UI also gives you the option to edit or even remove a secret, by hovering the secret you want to edit you will see these two icons:
+
+![Icons](./images/secrets-4.png)
+
+If you click on the pencil you will see it will turn into a form again where you can edit the name and value of your secret:
+
+![Icons](./images/secrets-5.png)
+
+This will take effect automatically and it will also restart your sandbox to make sure we use the new value.
+
+To delete you can click on the `x` icon after the pencil one and this one will also restart your sandbox.