README.md: Avoid sudo for scratch_link.py

The README instruction notes to run scratch_link.py with sudo. This is
not good for normal users especially for children. To allow normal users
to run scratch_link.py, revise the instruction.

Add a step to run setcap.sh at set up. Remove the step of btmgmt commands
with sudo which are not required when bluetooth.service is running.
Also add some more troubleshooting guides users might be caught at
scratch_link.py without sudo.

Signed-off-by: Shin'ichiro Kawasaki <kawasaki@juno.dti.ne.jp>

README.md

@@ -53,9 +53,9 @@ Installation
 
     ```sh
     Ubuntu
-    $ sudo apt install bluez libbluetooth-dev libnss3-tools
+    $ sudo apt install bluez libbluetooth-dev libnss3-tools libcap2-bin
     Arch
-    $ sudo pacman -S bluez bluez-utils nss
+    $ sudo pacman -S bluez bluez-utils nss libcap
     ```
 
 3. Install python modules.
@@ -94,7 +94,14 @@ Installation
     Added certificate to Chrome
     ```
 
-6. If using a micro:bit, install Scratch-link hex on your device.
+6. Set bluepy-helper capability
+    ```
+    ./setcap.sh
+    Set up bluepy-helper capability to allow use by normal users
+    /usr/lib/python3.8/site-packages/bluepy-1.3.0-py3.8.egg/bluepy/bluepy-helper = cap_net_admin,cap_net_raw+eip
+    ```
+
+7. If using a micro:bit, install Scratch-link hex on your device.
 
     * Download and unzip the [micro:bit Scratch Hex file](https://downloads.scratch.mit.edu/microbit/scratch-microbit-1.1.0.hex.zip).
     * Flash the micro:bit over USB with the Scratch .Hex File, you will see the
@@ -103,13 +110,7 @@ Installation
 
 Usage
 -----
-1. For micro:bit or other BLE devices, turn on Bluetooth Low Energy controller
-    ```sh
-    $ sudo btmgmt le on
-    $ sudo btmgmt power on
-    ```
-
-2. For Lego Mindstorms EV3, pair your Linux PC to the EV3 brick.
+1. For Lego Mindstorms EV3, pair your Linux PC to the EV3 brick.
 
    First, turn on the EV3 and ensure Bluetooth is enabled.
 
@@ -158,15 +159,15 @@ Usage
    [bluetooth]# quit
    ```
 
-3. Start scratch-link python script
+2. Start scratch-link python script.
     ```sh
     $ cd ~/bluepy-scratch-link
-    $ sudo ./scratch_link.py
+    $ ./scratch_link.py
     Or if your system has python3 command,
-    $ sudo python3 ./scratch_link.py
+    $ python3 ./scratch_link.py
     ```
 
-4. Connect scratch to micro:bit or Lego Mindstorms:
+3. Connect scratch to micro:bit or Lego Mindstorms:
     * Open FireFox or Chrome and access [Scratch 3.0](https://scratch.mit.edu/)
     * Select the "Add Extension" button
     * Select micro:bit or Lego Mindstorms EV3 extension and follow the prompts to connect
@@ -174,12 +175,21 @@ Usage
 
 In Case You Fail to Connect
 ---------------------------
-If Scratch says "Make sure you have Scratch Link installed" but you are sure
-that scratch-link python script is running...
 
-1. Check that Firefox or Chrome allow local server certificate
+1. If Scratch says "Make sure you have Scratch Link installed" but you are sure
+   that scratch-link python script is running, check that Firefox or Chrome
+   allows local server certificate.
     * Open Firefox or Chrome and access [https://device-manager.scratch.mit.edu:20110/](https://device-manager.scratch.mit.edu:20110/). You will see a security risk warning.
     * In **Firefox**: Click "Advanced" and click "Accept Risk and Continue".
     * In **Chrome**: type the special bypass keyword `thisisunsafe`.
     * Immediately, you will see "Failed to open a WebSocket connection". This is expected.
 
+2. If device scan fails, check systemd bluetooth service status.
+    ```
+    systemctl status bluetooth.service
+    ```
+    * If the service is not working, refer guide of your distro to set it up.
+    * If the service is working, also check that /etc/bluetooth/main.conf sets AutoEnable=true.
+
+3. If scratch_link.py says "failed to connect to BT device: [Errno 13] Permission denied",
+   make sure to pair the bluetooth device to your PC before connecting to Scratch.