README.md: Update to include building stdplus
README.md: Update to include building stdplus

There is a new dependency on stdplus library, need to include the steps
on installing the library in the README.md.

Resolves: openbmc/phosphor-ipmi-flash#3
Signed-off-by: Brandon Kim <dbcb4743d083bf663ac612bcd5bbeafe818a4d70@google.com>
Change-Id: I14822b65a5e7890458f974acf0bdd71016b0dd99

openbmc/phosphor-ipmi-flash