Mill, SBT and Scalafix docs added

Author: anatoliykmetyuk

docs/docs/contributing/tools/mill.md

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+Here's an example of how to test a project that uses mill:
+
+```bash
+mill utest.jvm[2.12.8].test
+```
+
+- `utest.jvm` - the name of the compiled module (obtain from `build.sc`)
+- `2.12.8` – Scala cross-compile version
+- `test` – task to run on the module specified with the specified Scala version
+
+To get mill of the most recent version, first, find it in https://github.com/lihaoyi/mill/releases (e.g. `0.4.2-1-020e28`). Copy the download link and substitute it in the following command instead of `https://github.com/lihaoyi/mill/releases/download/0.4.1/0.4.1`:
+
+```
+# From http://www.lihaoyi.com/mill/
+sudo sh -c '(echo "#!/usr/bin/env sh" && curl -L https://github.com/lihaoyi/mill/releases/download/0.4.1/0.4.1) > /usr/local/bin/mill && chmod +x /usr/local/bin/mill'
+```

docs/docs/contributing/tools/sbt.md

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+# SBT Commands Cheat Sheet
+The basics of working with Dotty codebase are documented [here](http://dotty.epfl.ch/docs/contributing/getting-started.html) and [here](http://dotty.epfl.ch/docs/contributing/workflow.html). Below is a cheat sheet of some frequently used commands (to be used from SBT console – `sbt`).
+
+
+|                        Command                       |                                                          Description                                                          |
+|------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------|
+| `dotc ../issues/Playground.scala`                    | Compile the given file – path relative to the Dotty directory. Output the compiled class files to the Dotty directory itself. |
+| `dotr Playground`                                    | Run the compiled class `Playground`. Dotty directory is on classpath by default.                                              |
+| `repl`                                               | Start REPL                                                                                                                    |
+| `testOnly dotty.tools.dotc.CompilationTests -- *pos` | Run test (method) `pos` from `CompilationTests` suite.                                                                        |
+| `testCompilation sample`                             | In all test suites, run test files containing the word `sample` in their title.                                               |

docs/docs/contributing/tools/scalafix.md

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+# Working with Scalafix
+
+First, create a new rule as follows (command from https://scalacenter.github.io/scalafix/docs/developers/setup.html):
+
+```bash
+sbt new scalacenter/scalafix.g8 --repo="Repository Name"
+```
+
+To run the rule against some codebase:
+
+```bash
+scalafix -r file:scalafix/rules/src/main/scala/fix/YourRule.scala your/code/base/
+```
+
+Where `YourRule.scala` is the rule you developed and `your/code/base` is the code base you are running the rule against.