Add Rails support to Solargraph (#342)

See:
- https://solargraph.org/guides/rails
- castwide/solargraph#87
- https://github.com/iftheshoefritz/solargraph-rails

.solargraph.yml

@@ -0,0 +1,23 @@
+---
+include:
+- "**/*.rb"
+exclude:
+- spec/**/*
+- test/**/*
+- vendor/**/*
+- ".bundle/**/*"
+require: []
+domains: []
+reporters:
+- rubocop
+- require_not_found
+formatter:
+  rubocop:
+    cops: safe
+    except: []
+    only: []
+    extra_args: []
+require_paths: []
+plugins:
+  - solargraph-rails
+max_files: 5000