add envvar middleware docs

SUMMARY.md

@@ -15,6 +15,7 @@
   * [CORS](api/middleware/cors.md)
   * [CSRF](api/middleware/csrf.md)
   * [Encrypt Cookie](api/middleware/encryptcookie.md)
+  * [EnvVar](api/middleware/envvar.md)
   * [ETag](api/middleware/etag.md)
   * [Expvar](api/middleware/expvar.md)
   * [Favicon](api/middleware/favicon.md)

api/middleware.md

@@ -24,6 +24,7 @@ description: >-
 * **gofiber/basicauth** Basic auth middleware provides an HTTP basic authentication. It calls the next handler for valid credentials and 401 Unauthorized for missing or invalid credentials.
 * **gofiber/cors** Enable cross-origin resource sharing \(CORS\) with various options.
 * **gofiber/csrf** Protect from CSRF exploits.
+* **gofiber/envvar** This middleware enables to expose environment variables.
 * **gofiber/helmet** Helps secure your apps by setting various HTTP headers.
 * **gofiber/jwt** JWT returns a JSON Web Token \(JWT\) auth middleware.
 * **gofiber/keyauth** Key auth middleware provides a key-based authentication.

api/middleware/envvar.md

@@ -0,0 +1,81 @@
+# Exposing Environment Variables Middleware
+
+EnvVar middleware for [Fiber](https://github.com/gofiber/fiber) that can be used to expose environment variables with various options.
+
+## Table of Contents
+
+* [Signatures](envvar.md#signatures)
+* [Examples](envvar.md#examples)
+	* [Default Config](envvar.md#default-config)
+	* [Custom Config](envvar.md#custom-config)
+	* [Response](envvar.md#response)
+* [Config](envvar.md#config)
+* [Default Config](envvar.md#default-config-1)
+
+## Signatures
+
+```go
+func New(config ...Config) fiber.Handler
+```
+
+## Examples
+
+First import the middleware from Fiber,
+
+```go
+import (
+  "github.com/gofiber/fiber/v2"
+  "github.com/gofiber/fiber/v2/middleware/envvar"
+)
+```
+
+Then create a Fiber app with `app := fiber.New()`.
+
+**Note**: You need to provide a path to use envvar middleware.
+
+### Default Config
+
+```go
+app.Use("/expose/envvars", envvar.New())
+```
+
+### Custom Config
+
+```go
+app.Use("/expose/envvars", envvar.New(envvar.Config{
+    ExportVars:  map[string]string{"testKey": "", "testDefaultKey": "testDefaultVal"},
+    ExcludeVars: map[string]string{"excludeKey": ""}}
+}))
+```
+
+### Response
+
+Http response contract:
+```
+{
+  "vars": {
+    "someEnvVariable": "someValue",
+    "anotherEnvVariable": "anotherValue",
+  }
+}
+
+```
+
+## Config
+
+```go
+// Config defines the config for middleware.
+type Config struct {
+    // ExportVars specifies the environment variables that should export
+    ExportVars map[string]string
+    // ExcludeVars specifies the environment variables that should not export
+    ExcludeVars map[string]string
+}
+
+```
+
+## Default Config
+
+```go
+Config{}
+```