Prominently documents 30GB disk space requirement

During review of #107, @dachary mentioned running out of disk space. We
used to be better about documenting that! Added to the requirements
section of the README, and cross-referenced from the build guide.

README.md

@@ -12,11 +12,15 @@ Build documentation can be found in the `docs/build.md` file.
 
 ## Requirements
 
+### Resources
 Only Debian and Ubuntu are supported, due to the use of make-kpkg.
 For compiling the kernel, 2GB and 2 VCPUs is plenty. Depending on the config options
 you specify, the compilation should take two to three hours on that hardware.
 Naturally, you can speed up the build by providing more resources.
 
+As for disk space, make sure you have at least 30GB to run the full kernel compile.
+
+### Credentials
 Furthermore, you must have a [grsecurity subscription] and export the
 following environment variables:
 

docs/build.md

@@ -7,7 +7,8 @@ and is intended for two use cases:
   * SecureDrop Admins recompiling a kernel from source
 
 Users trying to recompile grsecurity-patched kernels for contexts other
-than SecureDrop should look elsewhere.
+than SecureDrop should look elsewhere. Make sure to review the requirements
+in the README (such as 30GB free disk space) before continuing.
 
 ## Building the SecureDrop-specific packages for release
 The automated build logic for preparing source directories will streamline most