docs(readme): add the basic readme

README.md

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+## bitsvec
+
+A bit vector with [the Rust standard library's portable SIMD API](https://github.com/rust-lang/portable-simd).
+
+## How to use
+
+```rust
+let mut bitvec = BitVec::ones(1000); // create a bitvec contains 0 ..= 999
+bitvec.set(900, false); // delete 900 from bitvec
+bitvec.set(1200, true); // add 1200 to bitvec (and expand bitvec to length 1201)
+let bitvec2 = BitVec::ones(1000);
+
+let new_bitvec = bitvec.and_cloned(&bitvec2); // and operation, without consume
+let new_bitvec2 = bitvec & bitvec2; // and operation, consume both bitvec
+
+// Operation Supported:
+// and, or, xor, not, eq, eq_left
+
+assert_eq!(new_bitvec, new_bitvec2);
+```
+
+## Performance
+
+Compared on AMD Ryzen 9 5900hs, aginst:
+
+* [bit\_vec 0.6.3](https://docs.rs/bit-vec/0.6.3/bit_vec/index.html)
+* [bitvec 1.0.0](https://docs.rs/bitvec/1.0.0/bitvec/index.html)
+
+```
+$ cargo bench
+
+bitsvec(this crate)     time:   [348.51 ns 348.74 ns 348.98 ns]
+Found 7 outliers among 100 measurements (7.00%)
+  7 (7.00%) high mild
+
+bitsvec_u16x8(this crate)
+                        time:   [401.90 ns 403.02 ns 404.94 ns]
+Found 3 outliers among 100 measurements (3.00%)
+  1 (1.00%) low mild
+  2 (2.00%) high severe
+
+bit-vec 0.6             time:   [1.5891 us 1.5956 us 1.6034 us]
+Found 13 outliers among 100 measurements (13.00%)
+  6 (6.00%) high mild
+  7 (7.00%) high severe
+
+bitvec 1.0              time:   [32.228 us 32.516 us 32.821 us]
+Found 16 outliers among 100 measurements (16.00%)
+  13 (13.00%) low severe
+  3 (3.00%) high mild
+
+bitsvec(this crate) with creation
+                        time:   [884.09 ns 884.70 ns 885.35 ns]
+Found 6 outliers among 100 measurements (6.00%)
+  2 (2.00%) high mild
+  4 (4.00%) high severe
+
+bitsvec_u16x8(this crate) with creation
+                        time:   [878.66 ns 879.61 ns 880.66 ns]
+Found 4 outliers among 100 measurements (4.00%)
+  3 (3.00%) high mild
+  1 (1.00%) high severe
+
+bit-vec 0.6 with creation
+                        time:   [495.50 ns 495.87 ns 496.27 ns]
+Found 6 outliers among 100 measurements (6.00%)
+  2 (2.00%) high mild
+  4 (4.00%) high severe
+
+bitvec 1.0 with creation
+                        time:   [29.003 us 29.028 us 29.058 us]
+Found 15 outliers among 100 measurements (15.00%)
+  5 (5.00%) low severe
+  8 (8.00%) high mild
+  2 (2.00%) high severe
+```
+
+## Credits
+
+Most code of this crate is from (https://github.com/GCCFeli/bitvec_simd). On top of that, some changes were made.