path to *.h files to be included can be added in platformio.ini together with other dependencies. 
 See this example:
<code>
[env:nodemcuv2]
platform = espressif8266
board = nodemcuv2
framework = arduino
monitor_dtr = 0
monitor_rts = 0
monitor_speed = 9600
board_build.partitions = min_spiffs.csv
build_flags =
  -I "/PlatformIO/includes1"
  -I "/PlatformIO/includes2"
lib_deps =
  ESP8266WiFi
  ESP8266WebServer
  LittleFS
  ayushsharma82/ElegantOTA @ ^3.1.1
</code>

**elegantota_esp.h**
<code>
// under platformio.ini
board_build.partitions = min_spiffs.csv

// under lib_deps in platformio.ini
ESP8266WiFi
ESP8266WebServer
LittleFS
Ticker

// top of main.cpp
const char* ssid = "ssid";
const char* password = "password";
const char* hostname = "hostname";
bool nodemcuwificonnected = false;

// inside loop
server.handleClient();
ElegantOTA.loop();
</code>

**mqttasync.h**
<code>
// under platformio.ini 
marvinroger/AsyncMqttClient@^0.9.0

// under main.cpp
#define MQTT_HOST IPAddress(192, 168, x, x)
#define MQTT_PORT 1883
#define MQTT_TOPIC "topic"
</code>

**myfunc.h**
//Depending on use requirements//
<code>
// under platformio.ini
bblanchon/ArduinoJson @ ^7.2.1
pfeerick/elapsedMillis @ ^1.0.6
plageoj/UrlEncode @ ^1.0.1
</code>