Update cloud-vultr.md (trailofbits#1406)

* Update cloud-vultr.md

More fleshed-out instructions for generating an API key and saving the file. Also notes the default ansible behavior of looking for the file in `~/.vultr.ini`.

* Update README.md

Author: TC1977

README.md

@@ -204,6 +204,7 @@ After this process completes, the Algo VPN server will contain only the users li
   - Configure [Azure](docs/cloud-azure.md)
   - Configure [DigitalOcean](docs/cloud-do.md)
   - Configure [Google Cloud Platform](docs/cloud-gce.md)
+  - Configure [Vultr](docs/cloud-vultr.md)
 * Advanced Deployment
   - Deploy to your own [FreeBSD](docs/deploy-to-freebsd.md) server
   - Deploy to your own [Ubuntu 18.04](docs/deploy-to-ubuntu.md) server

docs/cloud-vultr.md

@@ -1,8 +1,15 @@
 ### Configuration file
 
-You need to create a configuration file in INI format with your api key (https://my.vultr.com/settings/#settingsapi)
+Algo requires an API key from your Vultr account in order to create a server. The API key is generated by going to your Vultr settings at https://my.vultr.com/settings/#settingsapi, and then selecting "generate new API key" on the right side of the box labeled "API Key". 
+
+Algo can read the API key in several different ways. Algo will first look for the file containing the API key in the environment variable $VULTR_API_CONFIG if present. You can set this with the command: `export VULTR_API_CONFIG=/path/to/vultr.ini`. Probably the simplest way to give Algo the API key is to create a file titled `.vultr.ini` in your home directory by typing `nano ~/.vultr.ini`, then entering the following text:
 
 ```
 [default]
 key = <your api key>
 ```
+where you've cut-and-pasted the API key from above into the `<your api key>` field (no brackets). 
+
+When Algo asks `Enter the local path to your configuration INI file
+(https://trailofbits.github.io/algo/cloud-vultr.html):` if you hit enter without typing anything, Algo will look for the file in `~/.vultr.ini` by default.
+