Added upgrading notes

UPGRADING-2.0.md

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+Upgrading from 1.x to 2.0
+=========================
+
+
+## BC breaking changes
+
+Native parameter and return types were added.
+As a consequence, some signatures were changed and will have to be adjusted in sub-classes. You can find a list of major changes to public API below.
+
+#### Doctrine\Common\Collections\Collection
+
+|             before             |                  after                         |
+|-------------------------------:|:-----------------------------------------------|
+| add($element)                  | add($element): bool                            |
+| clear()                        | clear(): void                                  |
+| contains($element)             | contains($element): bool                       |
+| isEmpty()                      | isEmpty(): bool                                |
+| removeElement($element)        | removeElement($element): bool                  |
+| containsKey($key)              | containsKey($key): bool                        |
+| getKeys()                      | getKeys(): array                               |
+| getValues()                    | getValues(): array                             |
+| set($key, $value)              | set($key, $value): void                        |
+| toArray()                      | toArray(): array                               |
+| exists(Closure $p)             | exists(Closure $p): bool                       |
+| filter(Closure $p)             | filter(Closure $p): self                       |
+| forAll(Closure $p)             | forAll(Closure $p): bool                       |
+| map(Closure $func)             | map(Closure $func): self                       |
+| partition(Closure $p)          | partition(Closure $p): array                   |
+| slice($offset, $length = null) | slice(int $offset, ?int $length = null): array |
+| count()                        | count(): int                                   |
+| getIterator()                  | getIterator(): \Traversable                    |
+| offsetSet($offset, $value)     | offsetSet($offset, $value): void               |
+| offsetUnset($offset)           | offsetUnset($offset): void                     |
+| offsetExists($offset)          | offsetExists($offset): bool                    |
+
+#### Doctrine\Common\Collections\AbstractLazyCollection
+
+|      before     |         after         |
+|----------------:|:----------------------|
+| isInitialized() | isInitialized(): bool |
+| initialize()    | initialize(): void    |
+| doInitialize()  | doInitialize(): void  |
+
+#### Doctrine\Common\Collections\ArrayCollection
+
+|            before           |               after               |
+|----------------------------:|:----------------------------------| 
+| createFrom(array $elements) | createFrom(array $elements): self |
+| __toString()                | __toString(): string              |