Use getrandom as default (for no-std feature)

.github/CONTRIBUTING.md

@@ -25,11 +25,17 @@ Take a look to the conventions established by existing code:
 * Every module should provide a Rust doctest, a brief test embedded with the documentation that explains how to use the procedure implemented.
 * Every module should provide comprehensive tests at the end, in its `mod tests {}` sub-module. These tests can be flagged or not with configuration flags to allow WebAssembly target.
 * Run `cargo doc --no-deps --open` and read the generated documentation in the browser to be sure that your changes reflects in the documentation and new code is documented.
+
+#### digging deeper
 * a nice overview of the codebase is given by [static analyzer](https://mozilla.github.io/rust-code-analysis/metrics.html):
 ```
 $ cargo install rust-code-analysis-cli
+// print metrics for every module
 $ rust-code-analysis-cli -m -O json -o . -p src/ --pr
+// print full AST for a module
+$ rust-code-analysis-cli -p src/algorithm/neighbour/fastpair.rs --ls 22 --le 213 -d > ast.txt
 ```
+* find more information about what happens in your binary with [`twiggy`](https://rustwasm.github.io/twiggy/install.html). This need a compiled binary so create a brief `main {}` function using `smartcore` and then point `twiggy` to that file.
 
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