Extract dependencies in README

extract dependencies to new headline. At the moment I have only added how to install dependencies for Ubuntu 20.04.

README.md

@@ -15,11 +15,11 @@ Currently the only solution is to set the password of `login` keyring to empty.
 
 I encrypt the `keyring-name : password` pair with GnuPG and save it as `secret-file`. Then on starting gnome, you have yubikey inserted. Then an auto-started script call GnuPG to decrypt the secret file, and pipe use the password to unlock your keyring. GnuPG will ask you to insert yubikey.
 
-## Usage
-
-> I recommend you to **configure Yubikey as GPG smartcard**. The system would just ask you to unlock gnome-keyring with your default GPG software. You may generate a new GPG key for yubikey, or move your existing GPG key into yubikey. Refer to google for these knowledge. 
+## Dependencies
+The project uses libgnome-keyring-dev
 
-First, install packages.
+### Ubuntu 20.04
+libgnome-keyring-dev is not in the repositories, you have to install it and its dependencies manually:
 ```
 wget http://archive.ubuntu.com/ubuntu/pool/main/g/glibc/multiarch-support_2.27-3ubuntu1_amd64.deb
 wget http://security.ubuntu.com/ubuntu/pool/universe/libg/libgnome-keyring/libgnome-keyring-common_3.12.0-1build1_all.deb
@@ -32,7 +32,11 @@ sudo dpkg-reconfigure multiarch-support
 sudo dpkg -i libgnome-keyring-common_3.12.0-1build1_all.deb libgnome-keyring0_3.12.0-1build1_amd64.deb gir1.2-gnomekeyring-1.0_3.12.0-1build1_amd64.deb libgnome-keyring-dev_3.12.0-1build1_amd64.deb
 ```
 
-Then, build the project from source.
+## Usage
+
+> I recommend you to **configure Yubikey as GPG smartcard**. The system would just ask you to unlock gnome-keyring with your default GPG software. You may generate a new GPG key for yubikey, or move your existing GPG key into yubikey. Refer to google for these knowledge. 
+
+First, build the project from source.
 ```
 git clone https://github.com/recolic/gnome-keyring-yubikey-unlock --recursive
 cd gnome-keyring-yubikey-unlock/src && make && cd ..