Added documentation for no_std build and fuzzing (#282)

* added unfinished no_std docs

* docs: added missing example

* Update no_std.md

Co-authored-by: Dominik Maier <domenukk@gmail.com>

docs/src/SUMMARY.md

@@ -19,3 +19,4 @@
 
 - [Advanced Features](./advanced_features/advanced_features.md)
   - [Concolic Tracing & Hybrid Fuzzing](./advanced_features/concolic/concolic.md)
+  - [Using LibAFL in no_std environments](./advanced_features/no_std/no_std.md)

docs/src/advanced_features/no_std/no_std.md

@@ -0,0 +1,35 @@
+# Using LibAFL in no_std environments
+
+It is possible to use LibAFL in `no_std` environments e.g. custom platforms like microcontrolles, kernels, hypervisors, and more.
+
+You can simply add LibAFL to your `Cargo.toml` file:
+
+```toml
+libafl = { path = "path/to/libafl/", default-features = false}
+```
+
+Then build your project e.g. for `aarch64-unknown-none` using
+```
+cargo build --no-default-features --target aarch64-unknown-none
+```
+
+## Use custom timing
+
+The minimum amount of input LibAFL needs for `no_std` is a monotonically increasing timestamp.
+For this, anywhere in your project you need to implement the `external_current_millis` function, which returns the current time in milliseconds.
+
+// Assume this a clock source from a custom stdlib, which you want to use, which returns current time in seconds.
+```c
+int my_real_seconds(void)
+{
+    return *CLOCK;
+}
+```
+and here we use it in Rust. `external_current_millis` is then called from LibAFL.
+Note that it needs to be `no_mangle` in order to get picked up by LibAFL at linktime.
+```rust
+#[no_mangle]
+pub extern "C" fn external_current_millis() -> u64 {
+    unsafe { my_real_seconds()*1000 }
+}
+```