update: lxml >= 4.3.4

ci: add python 3.8 for tests
doc: add information about lxml requirements

Closes #8

.travis.yml

@@ -2,6 +2,7 @@ language: python
 python:
   - '3.6'
   - '3.7'
+  - '3.8'
 install:
   - pip install -r requirements-test.txt
   - pip install -e .

README.md

@@ -10,9 +10,24 @@ RSS generator for python
 
 ## Installing
 
+genrss using lxml library that required:
+
+* libxml2 version 2.9.2 or later.
+* libxslt version 1.1.27 or later.
+
+To install the required development packages of these dependencies on Linux systems,
+use your distribution specific installation tool, e.g. apt-get on Debian/Ubuntu:
+
+```bash
+sudo apt-get install libxml2-dev libxslt-dev
+```
+
+
 Install and update using pip:
 
-``pip install -U genrss``
+```bash
+pip install -U genrss
+```
 
 
 ## A Simple Example

setup.py

@@ -19,7 +19,7 @@ if __name__ == '__main__':
         license='MIT',
         packages=['genrss'],
         install_requires=[
-            'lxml==4.3.4',
+            'lxml>=4.3.4',
             'pytz==2019.1'
         ]
     )