Add a README.md to explain a few things

test/README.md

+# ipfs whole tests using the [sharness framework](https://github.com/mlafeldt/sharness/)
+
+## Running all the tests
+
+Just use `make` in this directory to run all the tests.
+
+## Running just one test
+
+You can run only one test script by launching it like a regular shell
+script:
+
+```
+$ ./t0010-basic-commands.sh
+```
+
+## Sharness
+
+When running "make" in this directory for the first time, sharness
+will be downloaded from its github repo and installed in a "sharness"
+directory.
+
+Please do not change anything in the "sharness" directory.
+
+If you really need some changes in sharness, please fork it from
+[its cannonical repo](https://github.com/mlafeldt/sharness/) and
+send pull requests there.
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