add readme

[mwarning]

A small readme.

[mwarning]

A small explanation what on and off link prefix means is still missing.

[jplitza]

Good idea, thanks!

Two remarks:

• Please always use addresses and prefixes from 2001:db8::/32 ("db8", as in "debate") for documentation purposes
• The only difference between -p and -a is the L-flag! The A-flag is always set:
  

  uradvd/uradvd.c

  Lines 500 to 503 in 9b0da60

  	uint8_t flags = ND_OPT_PI_FLAG_AUTO;
  	if (G.prefixes_onlink[i])
  	flags |= ND_OPT_PI_FLAG_ONLINK;

  
  The L-flag means that the receiver is supposed to install a "connected route" for the prefix, while its absence tells the receiver to install a route via the announcing router for the prefix (if it doesn't already have a connected route for it). IMO it doesn't make much sense to set the A-flag without the L-flag, but oh well.

[mwarning]

Updated. Not sure if the explanation is slightly overkill. :D