Update workload projection key name constraints

Underscores are valid in secret key names so the constraint should be extended to include that. See https://kubernetes.io/docs/concepts/configuration/secret/#restriction-names-data

Signed-off-by: Tom Kennedy <kennedy513@gmail.com>

Author: tomkennedy513

README.md

@@ -202,7 +202,7 @@ A projected binding **MUST** be volume mounted into a container at `$SERVICE_BIN
 
 The projected binding **MUST** contain a `type` entry with a value that identifies the abstract classification of the binding.  It is **RECOMMENDED** that the projected binding also contain a `provider` entry with a value that identifies the provider of the binding.  The projected binding data **MAY** contain any other entry. Directories under `$SERVICE_BINDING_ROOT` that do not contain a `type` entry **SHOULD** be ignored by the workload process at runtime.
 
-The name of a binding entry file name **SHOULD** match `[a-z0-9\-\.]{1,253}`.  The contents of a binding entry may be anything representable as bytes on the file system including, but not limited to, a literal string value (e.g. `db-password`), a language-specific binary (e.g. a Java `KeyStore` with a private key and X.509 certificate), or an indirect pointer to another system for value resolution (e.g. `vault://production-database/password`).
+The name of a binding entry file name **SHOULD** match `[a-z0-9\-\._]{1,253}`.  The contents of a binding entry may be anything representable as bytes on the file system including, but not limited to, a literal string value (e.g. `db-password`), a language-specific binary (e.g. a Java `KeyStore` with a private key and X.509 certificate), or an indirect pointer to another system for value resolution (e.g. `vault://production-database/password`).
 
 The collection of files within the directory **MAY** change during the lifetime of the container or between container launches.