Add an API styleguide

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/README.md

@@ -8,6 +8,8 @@
 
 ## Styleguides
 
+- [API styleguide](api_styleguide.md) Use this styleguide if you are
+  contributing to the API.
 - [Documentation styleguide](doc_styleguide.md) Use this styleguide if you are
   contributing to documentation.
 - [SQL Migration Style Guide](migration_style_guide.md) for creating safe SQL migrations

doc/development/api_styleguide.md

+# API styleguide
+
+This styleguide recommends best practices for the API development.
+
+## Declared params
+
+> Grape allows you to access only the parameters that have been declared by your
+`params` block. It filters out the params that have been passed, but are not
+allowed.
+
+– https://github.com/ruby-grape/grape#declared
+
+### Exclude params from parent namespaces
+
+> By default `declared(params) `includes parameters that were defined in all
+parent namespaces.
+
+– https://github.com/ruby-grape/grape#include-parent-namespaces
+
+In most cases you will want to exclude params from the parent namespaces:
+
+```ruby
+declared(params, include_parent_namespaces: false)
+```
+
+### When to use `declared(params)`?
+
+You should always use `declared(params)` when you pass the params hash as
+arguments to a method call.
+
+For instance:
+
+```ruby
+# bad
+User.create(params) # imagine the user submitted `admin=1`... :)
+
+# good
+User.create(declared(params, include_parent_namespaces: false).to_h)
+```
+
+>**Note:**
+`declared(params)` return a `Hashie::Mash` object, on which you will have to
+call `.to_h`.
+
+But we can use directly `params[key]` when we access single elements.
+
+For instance:
+
+```ruby
+# good
+Model.create(foo: params[:foo])
+```
+
+>**Note:**
+Since you [should use Grape's DSL to declare params](doc_styleguide.md#method-description), [parameters validation and
+coercion] comes for free!
+
+[parameters validation and coercion]: https://github.com/ruby-grape/grape#parameter-validation-and-coercion