RFC: added performance tests for sparse getindex + a README file

In PR #7131 @ViralBShah suggested that I should add the `getindex` performance test to `perf/`.  Here a tidied up version of https://gist.github.com/mauro3/7f01931b5f25d7b397af.

Questions:
- I created a `README.md` as I tried to figure out how `perf/` works.  Maybe this could be useful?  It will need a read.
- more or fewer tests?
- which test should be included in `codespeed`?
I'd round out these descriptions a bit more.  Something along the lines of:

* `micro`: A set of micro-benchmarks commonly used to compare programming languages; these results are shown on [http://julialang.org/](http://julialang.org/)

* `blas`, `lapack`: Performance tests for linear algebra tasks from low-level operations such as matrix multiplies to higher-level operations like eigenvalue problems.

* `cat`: Performance tests for concatenation of vectors and matrices.

* `kernel`: Performance tests used to track real-world code examples that previously ran slowly.

* `shootout`: Tracks the performance of tests taken from the [Debian shootout](http://shootout.alioth.debian.org) performance tests.

* `sort`: Performance tests of sorting algorithms

* `spell`: Performance tests of Peter Norvig's spelling corrector

* `sparse`: Performance tests of sparse matrix operations.