test: add README with sharness test command coverage

See issue #906 (sharness test command coverage) that
discussed this.

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

test/README.md

+
+## Sharness test command coverage
+
+Module     |     Online Test |    Offline Test |
+-----------|-----------------|-----------------|
+object     |                 |           t0051
+ls         |                 |           t0045
+cat        |           t0040 |
+dht        |                 |
+bitswap    |                 |
+block      |                 |           t0050
+daemon     |           t0030 |             N/A
+init       |             N/A |           t0020
+add        |           t0040 |
+config     |           t0021 |           t0021
+version    |                 |           t0010
+ping       |                 |
+diag       |                 |
+mount      |           t0030 |
+name       |           t0110 |           t0100
+pin        |           t0080 |
+get        |           t0090 |           t0090
+refs       |           t0080 |
+repo gc    |           t0080 |
+id         |                 |
+bootstrap  |           t0120 |           t0120
+swarm      |                 |
+update     |                 |
+commands   |                 |