docs(generators:filter): create filter.md

[skip ci]

Author: Awk34

docs/generators/filter.md

@@ -0,0 +1,13 @@
+### Filter
+Generates a filter.
+
+Example:
+```bash
+yo angular-fullstack:filter myFilter
+[?] Where would you like to create this filter? client/app/
+```
+
+Produces:
+
+    client/app/myFilter/myFilter.filter.js
+    client/app/myFilter/myFilter.filter.spec.js