add readme and makefile

Makefile

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+.PHONY: test bench build
+
+all: test bench build
+
+tests:
+	@cargo test
+
+bench:
+	@cargo bench
+
+build:
+	@cargo build

README.md

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+# rsbloom
+
+A simple implementation of a Bloom filter, a space-efficient probabilistic data
+structure.
+
+## Bloom Filters
+
+A Bloom filter is a space-efficient probabilistic data structure that is
+used to test whether an element is a member of a set. It allows for queries
+to return: "possibly in set" or "definitely not in set". Elements can be
+added to the set, but not removed; the more elements that are added to the
+set, the larger the probability of false positives. It has been shown that
+fewer than 10 bits per element are required for a 1% false positive
+probability, independent of the size or number of elements in the set.
+
+The provided implementation allows you to create a Bloom filter specifying
+the approximate number of items expected to inserted and an optional false
+positive probability. It also allows you to approximate the total number of
+items in the filter.
+
+## Enhanced Double Hashing
+
+Enhanced double hashing is used to set bit positions within a bit vector.
+The choice for double hashing was shown to be effective without any loss in
+the asymptotic false positive probability, leading to less computation and
+potentially less need for randomness in practice by Adam Kirsch and
+Michael Mitzenmacher in [Less Hashing, Same Performance: Building a Better Bloom Filter](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.152.579&rep=rep1&type=pdf).
+
+The enhanced double hash takes the form of the following formula:
+
+g<sub>i</sub>(x) = (H<sub>1</sub>(x) + iH<sub>2</sub>(x) + f(i)) mod m, where
+
+H<sub>1</sub>
+is Murmur3 128-bit, H<sub>2</sub> is xxHash 64-bit, and f(i) = i<sup>3</sup>
+
+## Usage
+
+Add the `rsbloom` dependency to your `Cargo.toml`:
+
+```toml
+[dependencies]
+rsbloom = "0.1.0"
+```
+
+## Example
+
+```rust
+use rsbloom::BloomFilter;
+
+fn main() {
+  let approx_items = 100;
+  let mut bf = BloomFilter::new(approx_items);
+
+  bf.set(&"foo");
+  bf.set(&"bar");
+
+  bf.has(&"foo"); // true
+  bf.has(&"bar"); // true
+  bf.has(&"baz"); // false
+
+  bf.num_items_approx(); // 2
+}
+```
+
+## Tests
+
+```shell
+make test
+```
+
+## Benchmarks
+
+```shell
+make bench
+```