improve book docs based on @jaidi's input (#62)

src/getting_started/compiling_redox.md

@@ -26,6 +26,12 @@ If it does not work, try:
 $ make qemu kvm=no # we disable KVM
 ```
 
+or:
+
+```sh
+$ make qemu iommu=no
+```
+
 If this doesn't work either, you should go open an issue.
 
 Note

src/getting_started/preparing_the_build.md

@@ -13,13 +13,14 @@ Oh, you're not lazy? Well, then the next section is for you!
 
 Cloning the repository
 ----------------------
- 
+
  ```sh
- $ git clone https://github.com/redox-os/redox.git && cd redox && git submodule update init
+ $ git clone https://github.com/redox-os/redox.git --origin upstream --recursive && \
+    cd redox && git submodule update --recursive --init
  ```
- 
+
  Give it a while. Redox is big.
- 
+
 
 Installing the build dependencies manually
 ------------------------------------------