update readmes

exercises/crypto-square/README.md

 imtgdvsfearwermayoogoanouuiontnnlvtwttddesaohghnsseoau
 ```
 
-Output the encoded text in chunks.  Phrases that fill perfect rectangles
-`(r X c)` should be output `c` chunks of `r` length, separated by spaces.
-Phrases that do not fill perfect rectangles will have `n` empty spaces.
-Those spaces should be distributed evenly, added to the end of the last
-`n` chunks.
+Output the encoded text in chunks that fill perfect rectangles `(r X c)`,
+with `c` chunks of `r` length, separated by spaces. For phrases that are
+`n` characters short of the perfect rectangle, pad each of the last `n`
+chunks with a single trailing space.
 
 ```text
 imtgdvs fearwer mayoogo anouuio ntnnlvt wttddes aohghn  sseoau 

exercises/error-handling/README.md

 is rather programming language specific you'll have to refer to the tests
 for your track to see what's exactly required.
 
+# Java Tips
+
+This exercise requires you to handle exceptions. An [exception](https://docs.oracle.com/javase/tutorial/essential/exceptions/definition.html) is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
+
+In Java, there are two types of exceptions: checked and unchecked exceptions.
+
+- [Checked vs Unchecked Exceptions in Java](https://www.geeksforgeeks.org/checked-vs-unchecked-exceptions-in-java/)
+
+- [Unchecked Exceptions — The Controversy](https://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html)
+
+## Checked exceptions
+
+Checked exceptions are the exceptions that are checked at [compile time](https://en.wikipedia.org/wiki/Compile_time).
+
+### Practical implications
+
+You have to declare them in the [method signature](https://docs.oracle.com/javase/tutorial/java/javaOO/methods.html) of any method that can [throw](https://docs.oracle.com/javase/tutorial/essential/exceptions/throwing.html) a checked exception and handle or rethrow them when calling any method that can throw a checked exception.
+
+This is because checked exceptions are meant to be handled at runtime, i.e. they are errors you can recover from.
+
+### Examples of where they are used
+
+They're often used when a method can't return any valid result, for example a search method which hasn't found the item it was searching for.
+
+It's an alternative to returning null or a error code. A checked exception is better than those alternatives because it forces the user of the method to consider the error case.
+
+## Unchecked exceptions
+
+Unchecked exceptions are the exceptions that are not checked at [compile time](https://en.wikipedia.org/wiki/Compile_time).
+
+### Practical implications
+
+You don't have to declare them in the [method signature](https://docs.oracle.com/javase/tutorial/java/javaOO/methods.html) of methods that can [throw](https://docs.oracle.com/javase/tutorial/essential/exceptions/throwing.html) an unchecked exception and handle or rethrow them when calling any method that can throw an unchecked exception.
+
+### Examples of where they are used
+
+Unchecked exceptions are mean to be used for any error than can't be handled at runtime, e.g. running out of memory.
+
 # Running the tests
 
 You can run all the tests for an exercise by entering

exercises/series/README.md

 # Series
 
 Given a string of digits, output all the contiguous substrings of length `n` in
-that string.
+that string in the order that they appear.
 
 For example, the string "49142" has the following 3-digit series:
 
-- 491
-- 914
-- 142
+- "491"
+- "914"
+- "142"
 
 And the following 4-digit series:
 
-- 4914
-- 9142
+- "4914"
+- "9142"
 
 And if you ask for a 6-digit series from a 5-digit string, you deserve
 whatever you get.

exercises/sieve/README.md

 
 The Sieve of Eratosthenes is a simple, ancient algorithm for finding all
 prime numbers up to any given limit. It does so by iteratively marking as
-composite (i.e. not prime) the multiples of each prime,
-starting with the multiples of 2.
+composite (i.e. not prime) the multiples of each prime, starting with the
+multiples of 2. It does not use any division or remainder operation.
 
 Create your range, starting at two and continuing up to and including the given limit. (i.e. [2, limit])
 
 
 Notice that this is a very specific algorithm, and the tests don't check
 that you've implemented the algorithm, only that you've come up with the
-correct list of primes.
+correct list of primes. A good first test is to check that you do not use
+division or remainder operations (div, /, mod or % depending on the
+language).
 
 # Running the tests