doc(constraints): Added in ConsulCatalog backend + new 'Constraint' section

docs/toml.md

@@ -195,6 +195,51 @@ entryPoint = "https"
    main = "local4.com"
 ```
 
+## Constraints
+
+In a micro-service architecture, with a central service discovery, setting constraints limits Træfɪk scope to a smaller number of routes.
+
+Træfɪk filters services according to service attributes/tags set in your configuration backends.
+
+Supported backends:
+
+- Consul Catalog
+
+Supported filters:
+
+- ```tag```
+
+```
+# Constraints definition
+
+#
+# Optional
+#
+
+# Simple matching constraint
+# constraints = ["tag==api"]
+
+# Simple mismatching constraint
+# constraints = ["tag!=api"]
+
+# Globbing
+# constraints = ["tag==us-*"]
+
+# Backend-specific constraint
+# [consulCatalog]
+#   endpoint = 127.0.0.1:8500
+#   constraints = ["tag==api"]
+
+# Multiple constraints
+#   - "tag==" must match with at least one tag
+#   - "tag!=" must match with none of tags
+# constraints = ["tag!=us-*", "tag!=asia-*"]
+# [consulCatalog]
+#   endpoint = 127.0.0.1:8500
+#   constraints = ["tag==api", "tag!=v*-beta"]
+```
+
+
 # Configuration backends
 
 ## File backend
@@ -741,6 +786,13 @@ domain = "consul.localhost"
 # Optional
 #
 prefix = "traefik"
+
+# Constraint on Consul catalog tags
+#
+# Optional
+#
+constraints = ["tag==api", "tag==he*ld"]
+# Matching with containers having this tag: "traefik.tags=api,helloworld"
 ```
 
 This backend will create routes matching on hostname based on the service name