Readme: Add requirements

README.adoc

@@ -13,7 +13,10 @@ TODO
 
 == Requirements
 
-TODO
+* Linux system
+* http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html[POSIX-sh] compatible shell (e.g. Busybox ash, dash, ZSH, bash, …)
+* common userland with `chroot`, `getopt`, `grep`, `sed`, `sha256sum`, `wget` (e.g. from Busybox, GNU coreutils, …)
+* `tar` (full-blown)
 
 
 == Usage