draft sdk readme

for those who've asked so many times, here's what will be in the SDK
[a draft 1 page readme file].

if anyone thinks of something the readme doesn't refer to that should
be in the SDK, please let me know asap [i'm working on a draft of the
philosophy doc atm, basically describing the location independence, anonymity,
 security, etc stuff from a high level, app dev perspective (not from
a i2p router dev perspective)].  comments/concerns/questions are always
welcome.

btw, the sdk 0.1 will probably be a quiet release (i2p dev, #iip-dev
people, and anyone who asks on #iip-dev for it).  no publicity until
1.0

-jrandom