Direct the user to python-ldap installation docs in the README

As demonstrated by the 230 upvotes and 180k+ views on
https://stackoverflow.com/questions/4768446/i-cant-install-python-ldap
- it doesn't usually install the first time.

As the README didn't mention the prerequisites, I immediately turned to
Google after encountering an error, which was less helpful than the
official docs.

(I moved the Python compatibility info to another section, as it looked
out of place after I added the link to python-ldap installation
instructions.)

Author: Nickolay Ponomarev

README.md

@@ -5,6 +5,8 @@ Flask-SimpleLDAP
 
 Flask-SimpleLDAP provides LDAP authentication for Flask.
 
+Flask-SimpleLDAP is compatible with and tested on Python 3.5, 3.6 and 3.7.
+
 Quickstart
 ----------
 
@@ -13,8 +15,8 @@ First, install Flask-SimpleLDAP:
     $ pip install flask-simpleldap
 
 Flask-SimpleLDAP depends, and will install for you, recent versions of Flask
-(0.12.4 or later) and [python-ldap](https://python-ldap.org/). Flask-SimpleLDAP is compatible
-with and tested on Python 3.5, 3.6 and 3.7.
+(0.12.4 or later) and [python-ldap](https://python-ldap.org/).
+Please consult the [python-ldap installation instructions](https://www.python-ldap.org/en/latest/installing.html) if you get an error during installation.
 
 Next, add a ``LDAP`` instance to your code and at least the three
 required configuration options: