Added README

Author: octavonce

README.md

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+A drop-in global allocator wrapper around the [mimalloc](https://github.com/microsoft/mimalloc) allocator.
+Mimalloc is a general purpose, performance oriented allocator built by Microsoft.
+
+## Usage
+```rust
+use mimalloc::MiMalloc;
+#[global_allocator]
+static GLOBAL: MiMalloc = MiMalloc;
+```
+
+## Usage without secure mode
+By default this library builds mimalloc in safe-mode. This means that
+heap allocations are encrypted, but this results in a 3% increase in overhead.
+
+In `Cargo.toml`:
+```rust
+[dependencies]
+mimalloc = { version = "*", features = ["no_secure"] }
+```