README: fix markdown lints

Headings and lists should be surrounded by empty lines.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

Author: lsf37

README.md

@@ -19,7 +19,9 @@ seL4test (this project).
 *Small unit tests* can be defined anywhere, such as libraries outside of `sel4test` or in `sel4test-driver`. *Larger tests* that do things like creating processes need to be declared inside `sel4test-tests`.
 
 ### Unit tests
+
 To define a small unit test in a library outside of `sel4test` or in `sel4test-driver`:
+
 1. Declare `libsel4test` as a dependency for your library and include `<sel4test/test.h>`. You may also find the functions in `<sel4test/testutil.h>` handy.
 2. Write your tests. Then, for each test you want to run, call one of the macros that define a test, such as the `DEFINE_TEST` macro. They are declared
 [here](https://github.com/seL4/seL4_libs/blob/master/libsel4test/include/sel4test/test.h#L88).
@@ -30,11 +32,14 @@ Add a call to any function in your test file to `testreporter.c` in [`dummy_func
 For an example, take a look at [`libsel4serialserver/src/test.c`](https://github.com/seL4/seL4_libs/blob/master/libsel4serialserver/src/test.c) in `sel4_libs`.
 
 ### Assumptions
+
 Currently unit tests are assumed to be running sequentially, standalone (i.e. not multi-threaded).
 Some tests rely on being the highest priority running thread in the system.
 
 ### Other tests
+
 To define a larger test in `sel4test-tests`:
+
 1. Place your test in `apps/sel4test-tests/src/tests`.
 2. Include `<../helpers.h>`.
 3. Write your tests. Then, for each test you want to run, call one of the macros that define a test,