DOC: How to fix the installation issue that Serd cannot be found.

This issue was also notices by @ColinMaudry.

Author: wouterbeek

README.md

@@ -59,6 +59,38 @@ make -j2
 sudo make install
 ```
 
+### Installation issues
+
+Sometimes, the above instructions do not result in a working HDT
+installation.  This section enumerates common issues and their
+workaround.
+
+#### `./configure` cannot find Serd
+
+While running `./configure` you get a message similar to the
+following:
+
+```
+Package 'serd-0', required by 'virtual:world', not found
+```
+
+This means that `./configure` cannot find the location of the
+`serd-0.pc` file on your computer.  You have to find this location
+yourself, e.g., in the following way:
+
+```sh
+find /usr/ -name serd-0.pc
+```
+
+Once you have found the directory containing the `serd-0.pc` file, you
+have to inform the `./configure` script about this location by setting
+the following environment variable (where directory
+`/usr/local/lib/pkgconfig/` is adapted to your situation):
+
+```sh
+export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig/
+```
+
 ## Using HDT
 
 After compiling and installing, you can use the handy tools that are