Created Readme to document usage and functions

README.md

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+# Clockodo.NET
+.NET library for the time and attendance tracking software Clockodo
+
+# Features
+Clockodo.NET does currently support the following Clockodo API functionality:
+
+- Time Entries Endpoint
+- Grouping and filtering of time entries
+
+# Getting Started
+To use this library you have to generate an API Key. Log into Clockodo and click on the profile icon in the top right. Then click on `My Data` and scroll down to `API-Key`. Click on `Generate API-Key` and copy it.
+
+# Usage
+First you have to create a new instance of ClockodoService with your API token and additional information like this:
+```csharp
+var clockodoCredentials = new Credentials(
+  "[email protected]",
+  "your-api-token",
+  "application-name", // You can choose any name you like
+  "contact-email"     // Does not have to be the account E-Mail
+);
+
+var clockodoService = new ClockodoService(clockodoCredentials);
+```
+The application name and contact E-mail can be whatever you like. They dont have to be connected to your account or setup in clockodo. Clockodo uses it for `Client-Identification`.
+
+## Getting all time entries
+
+### Creating a Period
+To get all time entries you first have to choose which period you want to query the time entries from.
+
+The `ClockodoPeriod` class only allows for the creation of valid periods. The end date cannot be earlier than the start for example. It returns an object of the type `Result`.
+
+```csharp
+var periodStart = new DateTime(2021, 01, 01);
+var periodEnd = new DateTime(2021, 05, 31);
+
+var periodResult = ClockodoPeriod.Create(periodStart, periodEnd);
+
+if (periodResult.IsFailure)
+{
+    Console.WriteLine(periodResult.Error);
+    return;
+}
+
+var period = periodResult.Value;
+```
+
+### TimeEntry and RawTimeEntry
+Then you can either use the `GetRawTimeEntriesAsync` or `GetTimeEntriesAsync` function provided by the `ClockodoService` instance.
+
+The difference between the `RawTimeEntry` and the `TimeEntry` class is that the `TimeEntry` class already parses dates and conducts null checks, while the `RawTimeEntry` class only contains primitive times that might also be nullable.
+
+```csharp
+// Raw data with priimitive types
+var rawTimeEntries = await clockodoService.GetRawTimeEntriesAsync(period);
+
+// Parsed objects
+var timeEntries = await clockodoService.GetTimeEntriesAsync(period);
+```
+
+### Filters
+Clockodo.NET also provides filter functionality for instances of `TimeEntry`. 
+
+There is an `EmployeeFilter` and a `YearFilter`. Those filters can also be combined as needed. 
+
+In the following example we want to get a list of all time entries `John` made in the year `2021`:
+
+```csharp
+var filters = new List<IFilterTimeEntries>
+{
+  new EmployeeFilter(employee),
+  new YearFilter(year)
+};
+
+var filteredEntries = timeEntries.ApplyAllFilters(filters);       
+```
+
+### Grouping
+You can also group time entries by employee and customer. This will aggregate the time entries each employee has created for a each customer he worked with.
+
+```csharp
+var groupedEntries = timeEntries.GroupByEmployeeAndCustomer();
+```