Added build step back in, added update section

Due to a cached build on my test machine and a general lack of golang knowledge, I didn't specify the steps correctly but had the infamous "works on my machine" kind of result.  I've tested these ones better by clearing out all traces of the module between runs.

README.md

@@ -32,6 +32,7 @@ Clone and build the repository
 
 ```sh
 go get github.com/akshaykarle/terraform-provider-mongodbatlas
+go build github.com/akshaykarle/terraform-provider-mongodbatlas
 ```
 
 Symlink the binary to your terraform plugins directory:
@@ -40,5 +41,12 @@ Symlink the binary to your terraform plugins directory:
 ln -s $GOPATH/bin/terraform-provider-mongodbatlas ~/.terraform.d/plugins/
 ```
 
+## Updating the Provider
+
+```sh
+go get -u github.com/akshaykarle/terraform-provider-mongodbatlas
+go build github.com/akshaykarle/terraform-provider-mongodbatlas
+```
+
 ## NOTE
 The `mongodbatlas_project` and `mongodbatlas_container` resources do not destroy the project or container (vpc) in mongo atlas. This due to limitation of the mongo atlas API as it doesn't support deleting these resources.