Add doc for checkers layer

docs/layers/checkers.md

@@ -0,0 +1,14 @@
+# [Layers](https://spacevim.org/layers) > checkers
+
+This layer provides syntax checking feature.
+
+## Install
+
+checkers layer is loaded by default.
+
+## options
+
+Name | default value | description
+---- | ------------- | -----------
+g:spacevim_enable_neomake | 1 | Use neomake as default checking tools, to use syntastic, set this option to 0.
+g:spacevim_lint_on_the_fly | 0 | Syntax checking on the fly feature, disabled by default.