add config

config/source/env/README.md

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+# Env Source
+
+The env source reads config from environment variables
+
+## Format
+
+We expect environment variables to be in the standard format of FOO=bar
+
+Keys are converted to lowercase and split on underscore.
+
+
+### Example
+
+```
+DATABASE_ADDRESS=127.0.0.1
+DATABASE_PORT=3306
+```
+
+Becomes
+
+```json
+{
+    "database": {
+        "address": "127.0.0.1",
+        "port": 3306
+    }
+}
+```
+
+## Prefixes
+
+Environment variables can be namespaced so we only have access to a subset. Two options are available:
+
+```
+WithPrefix(p ...string)
+WithStrippedPrefix(p ...string)
+```
+
+The former will preserve the prefix and make it a top level key in the config. The latter eliminates the prefix, reducing the nesting by one. 
+
+#### Example:
+
+Given ENVs of:
+
+```
+APP_DATABASE_ADDRESS=127.0.0.1
+APP_DATABASE_PORT=3306
+VAULT_ADDR=vault:1337
+```
+
+and a source initialized as follows:
+
+```
+src := env.NewSource(
+    env.WithPrefix("VAULT"),
+    env.WithStrippedPrefix("APP"),
+)
+```
+
+The resulting config will be:
+
+```
+{
+    "database": {
+        "address": "127.0.0.1",
+        "port": 3306
+    },
+    "vault": {
+        "addr": "vault:1337"
+    }
+}
+```
+
+
+## New Source
+
+Specify source with data
+
+```go
+src := env.NewSource(
+	// optionally specify prefix
+	env.WithPrefix("MICRO"),
+)
+```
+
+## Load Source
+
+Load the source into config
+
+```go
+// Create new config
+conf := config.NewConfig()
+
+// Load file source
+conf.Load(src)
+```