full-trace.sh: check if the mandatory argument has been given

The full-trace.sh script needs to know which command is to be
traced. However, no check is performed on the presence of such
an argument. This results in non-explicative errors during the
execution of the script. This patch adds a check on the presence
of this mandatory argument and possibly notifies the user about
its absence, printing also the script usage.
README.md: fix TOC and add a paragraph on ftrace buffer sizing

The README.md file contains a table of contents, whose bullets are
links to the specific paragraphs. The file used anchor tags to define
the internal hyperlinks; however, the special github Markdown plugins
automatically define anchor points for each paragraph. This patch
attempts to adapt the TOC to play along nicely with the auto-generated
anchor points.
This patch also adds a brief paragraph and an usage example for the
-b option of the full-trace.sh script, that allows the user to specify
the per-CPU ftrace buffer.