Document the .mbedignore file

docs/ignoring_files_from_build.md

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+# Ignoring files from mbed build
+
+The `.mbedignore` file allows you to ignore files and directories from being processed by `mbed build` command.
+
+## Usage
+You can place the `.mbedignore` file in any directory where `mbed build` command is going to search for source files.
+
+The most convenient place is the root directory of the library or application. However, this is not a requirement.
+
+Avoid defining rules that would cross the library boundaries as those would lead to side effects or build problems that are hard to find.
+
+## Syntax
+
+Each line in the `.mbedignore` file is a file pattern used for matching files. Each matched file or directory is ignored from build.
+
+The following wildcards are accepted:
+
+|Pattern | Meaning|
+|--------|--------|
+| `*` | Matches everything. |
+| `?` | Matches any single character. |
+| `[seq]` | Matches any character in seq. |
+| `[!seq]` | Matches any character not in seq. |
+
+File is parsed with Python's [fnmatch](https://docs.python.org/2/library/fnmatch.html) functionality so the syntax follows basic shell patterns with the following exceptions:
+
+1. Each line is internally prefixed with the path of the `.mbedignore` file.
+2. Line cannot start with `.` or `/` (because of rule 1)
+
+Globbing functionality is not used, so you cannot recursively match specific file pattern. You need to define rule per directory in that case.
+
+Relative paths can be used, so you can match files deeper in the build tree. However, avoid crossing library boundaries.
+
+### Example
+A file located in `source/obsolete/.mbedignore` with following content:
+
+```
+*.c
+*.h
+second_level/*.c
+```
+
+After applying the rule 1, actual patterns used internally for matching the source files are:
+
+```
+source/obsolete/*.c
+source/obsolete/*.h
+source/obsolete/second_level/*.c
+```