get_tld(), get_components(), and other

UseCases.md

@@ -0,0 +1,106 @@
+This file describes exact behavior of methods for different edge cases and 
+explains general logic. This description covers the behavior of get_tld, 
+get_tld_unsafe, get_sld, get_sld_unsafe, split_domain, split_domain_unsafe
+
+Unsafe versions of the methods will significantly save resources on large-scale 
+applications of the library where the data has already been converted to 
+lowercase and missing data has a None value. This can be done in Spark/Dask, 
+for example, and result in a significant reduction in computational resources. 
+For adhoc usage, the original functions are sufficient.
+
+1. general difference of get_*() and get_*_unsafe() methods:
+get_*_unsafe() does not perform if the input string is None and does not 
+transforms it to the lower case.
+
+2. The listed above methods works only with non-canonical FQDN strings - 
+trailing dot must be removed before call the method. This restriction allows 
+get rid of fuzzy logic in edge cases.
+
+3. DNS does not support empty labels - if some label detected to be empty, 
+None will be returned. 
+
+4. Every method processes provided FQDN in the reverse order, from the last 
+label towards the start of the string. It stops when the specific task is 
+completed. Therefore no validation occurs outside of this scope.  For example,
+```
+get_tld('......com') -> 'com'
+``` 
+as leading dots are not processed.
+split_domain method is based on get_sld method - it returns everything in 
+front of get_sld() as a prefix.
+Specifically to example above 
+```
+split_domain('......com') -> ('....',None,'com')
+```
+Edge cases and expected behavior
+The behavior of the library can be illustrated best on the small examples:
+(boolean arguments are omitted if does not affect behavior )
+